Here's what I've got.
These might be more wordy than they need to be, sorry! It's mostly all the matchups.
Entries for Hoppip, Remoraid, and Oddish:
Hoppip for
Bottom.
Hoppip
Availability: Early. Routes 32 and 33, right after Violet City.
Stats: Speed is great! Unfortunately, that's all it really has. Its attack stats are very low and its defenses are only slightly higher, and the attack difference gets larger and larger as more things start evolving. Jumpluff is one of the fastest things in the game, however.
Movepool: Horrible. Let me put it this way: It doesn't even get damage until level 10. And that's Tackle. It does get Synthesis extremely early at either the start or level 5(Crystal) but without evolving it doesn't learn a single grass move until Mega Drain at level
30,
36, or
44. Between them(early teens and level 22 or so) it gets the standard Spore/Powder/Powder Trifecta and Leech Seed, meaning it can conceivably Leech/Poison or Leech/Sleep stall things to death with Synthesis.
Gyms that it fares well against:
Oh god, here we go. Brock is about the only major fight Hoppip can handle.
Falkner: No. It probably can't even attack yet.
Bugsy: It can't even kill Metapod, let alone Kakuna. Scyther can't get a super effective hit on it, but that doesn't matter because Quick Attack kills it quickly anyway. Tackle won't help.
Miltank: You probably have either Stun Spore or Sleep Powder by now and those can help with Miltank, but that's about it.
Morty: You can't hurt them and they can hurt you. You can toss a leech seed onto something as well as status, but they'll take you down easily. Skiploom is really only slightly better than Hoppip.
Chuck: Well, you resist Fighting and Surf. Too bad you don't have any Grass or Flying moves to use on Poliwrath, and your lame defenses mean that both Primeape and Wrath can probably take you down quickly anyway.
Jasmine: You can't kill the Magnemites without the Leech Spam strategy, and you don't resist their attacks either. You also can't try to hurry things up with Poisonpowder. Steelix in particular kills you.
Pryce: Really, really, really don't want to be here.
Clair: You can status something. Just... don't get hit. At all. Especially by Blizzard.
Will: Meh. You probably finally have Mega Drain(and maybe Solarbeam), for all it's worth, but Will's team is surprisingly vicious to Grass types. You can try to Mega Drain/Solarbeam Slowbro but he'll probably win with Amnesia.
Koga: Ariados can't hurt you much, but you can't hurt him much back. Venomoth is able to kill you, you can't hurt Forretress, and Muk and Crobat can actually get a super effective hit on you. Not to mention that Crobat is actually faster than/as fast as you.
Bruno: You can kill Onix. You resist everyone else, but Hitmonchan can actually do fatal damage with Ice Punch and you can't really kill any of them in a reasonable time frame.
Karen: Vileplume acids you, Gengar curses you, Houndoom blows you out of the water, you can't hurt Murkrow... the only thing you're even sort of good at here is that you're actually more irritating than Umbreon and can win the annoy off with Leech Seed.
Lance: No. Just no. Everything here hits you super effectively, or so hard it doesn't matter.
Lt. Surge: Neutral hits can still kill Jumpluff without trouble, and Electrode is faster. Stay out.
Sabrina: Psychic kills you. Maybe you can sleep something.
Misty: You can kill Quagsire, but Starmie and Lapras have Ice moves. Stay away.
Erika: Leech Seed doesn't work on grass types, and since your only other trick is Mega Drain you should just back off. Shame you don't get any Flying type attacks.
Janine: They resist Grass and their STAB KOs you. Just stay out.
Brock: You can do surprisingly well here by just spamming Giga Drain/Solarbeam for all it's worth. You're much faster. Just watch for incoming Rock attacks.
Blaine: All of his team can take a hit and kill you without care. Stay out.
Blue: Eeeeeh. You can kill Rhydon if you 1HKO, but Rock Slide will KO you back. Nothing else is weak enough to die to you.
Red: You can try to Giga Drain Blastoise but it will just Blizzard you and a Sunnybeam combo will fail because it has Rain Dance. If you're really feeling up to it, you can try to Leech Seed Snorlax and switch to someone who can actually handle him. Just don't put him to sleep.
Other info: Hoppip is really weak and almost completely useless. Skiploom is nearly as bad, but Jumpluff at least has speed on its side. It dies to Team Rocket. It dies to half the gyms and can't make a dent in the others. It's incapable of dealing even middling damage most of the time, and it doesn't get any good STAB... basically ever. It's really kind of a shame, but such is the life of a Johto Grass type. Compared to other Grasses, it might be better than Paras, but that's a long shot. It is by far one of the least useful early pokemon.
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Remoraid for
Bottom.
Remoraid
Availability: Late. Remoraid is very late by Johto standards and possibly won't be obtainable until Kanto at level 40. In order to catch one you need to fish at Route 44, either with a Super Rod or when Remoraid is swarming(Fisherman Wilton will call you if so). Due to the unlikeliness of swarms, I'm going to have to say his availability is very low. It also doesn't exist in Crystal.
Stats: Remoraid is a glass cannon with only mediocre attack stats at the time you get it, especially since it'll probably be about ten levels lower than the rest of your team. Once it reaches level 25(Fairly easy and if you're catching it late then it'll be level 40 anyway) it evolves and becomes Octillery, who is much slower but has vaguely decent defenses as well as very good balanced offenses.
Movepool: Below average. If swarming, Remoraid will gain a trifecta of Aurora Beam/Psybeam/Bubblebeam(replace this with Surf) at level 22 that will help it deal damage to many foes (though sadly it comes right after the last time Team Rocket is a problem). It also comes with Lock On when caught. However, its TM movepool is pathetic and the only real coverage option it gets is Thief. It can be used for Whirlpool if necessary, though. It is notable for learning Ice Beam naturally, but if you evolve it on time (otherwise its somewhat useless) you'll have to wait until level /54/ to use it. Remoraid can be left unevolved to get it at level 44 in time for the Elite Four, but that delegates it to being severely dead weight for 20 levels.
Gyms that it fares well against:
Remoraid comes late and really doesn't do so well, especially since it's underleveled.
Clair: If you took the time to train him up, he might be able to Aurora Beam the Blizzard Dragonair. Unfortunately, he can't do much to anything else, and is too slow to do anything before dying.
Will: Octillery is meh and takes a lot of investment, but can be trained as a Remoraid on the common rock/grounds in victory road in order to get Ice Beam before evolving. A level 44 Octillery with Ice Beam can take a Psychic and kill the Xatus. If it's been taught Thief, it might also be able to kill Jynx and Slowbro, but I wouldn't recommend it. Exeggutor doesn't have any Grass moves, but Octillery isn't really tanky and an Egg Bomb will pulverize its soft, fleshy body.
Koga: Ariados has Giga Drain but also has really bad Special Attack, so conceivably you could Psybeam it to death. I really don't think it's a good idea, though. You might be able to take on Crobat, but he hits pretty hard and you don't want to be Toxiced.
Bruno: Not fun. Onix dies, yeah, but everything else can hit you pretty heavily. You might have Psybeam on you, but even Hitmonchan's pathetic elemental punches might constitute a problem.
Karen: Somewhat decent, oddly enough. Vileplume will murder you because you're too slow, but Gengar can't really hurt you and is only dangerous because of Destiny Bond(try Psybeam). Houndoom is deadly but Surf off your Special Attack is pretty good. Murkrow can't kill you before Ice Beam kills it. Umbreon is sort of meh. You don't really have the power to knock it out fast, so it'll annoy you to death.
Lance: Same as Clair. You can kill the Blizzard 'nite and maybe Aerodactyl. Charizard is a possibility if it can't get off Slash before you kill it, but everything else here is bad news.
Lt. Surge: No.
Sabrina: Thief isn't good enough and nothing here has a secondary type to exploit. No.
Misty: Everything here is a better water type than you. It may seem you can abuse Psybeam's neutrality on them, but it's not very strong at all and all of them have a way to mess you up (Golduck can disable Psybeam, Quagsire can Amnesia up and strike back with Earthquake, Lapras has Perish Song, and Starmie has recover) so you're not really usable.
Erika: Grass moves are your nemesis, and because they don't show up in Crystal it can't even attempt to alleviate it with Flamethrower(not that it would). Even Jumpluff will take you down.
Janine: Janine is low level enough that you can probably Surf sweep, but watch out for Giga Drain from Ariados. Luckily, they're not all that fast either.
Brock: Eh. Surf takes down the 4x weaks, but you're better off just switching for the fossils.
Blaine: Surf kills this team, but Arcanine and Rapidish might soften you up enough to kill you. Magcargo wins a "worst fire type" award for being slower than you are.
Blue: Yeeeeeeaaah not great. You can kill Rhydon, and maybe Arcanine. Nothing else except a possible Ice Beam kill for Pidgeot.
Red: You should be able to kill Charizard. Unfortunately, everything else will kill you, fast.
Other info: Octillery is probably one of the worst fully evolved water types in the game. It comes late, is easy to miss, doesn't even have any good moves for the time it does show up and is dangerously easy to KO considering its lame speed. It does get acceptable coverage for some of the smaller fights, but realistically you'd be better off with Wooper or something.
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Oddish for
Low.
Oddish
Availability: Fairly early; Oddish is first found when you reach Ilex Forest, and is fairly common at night.
Stats: Good early on, but suffer after about Olivine. Oddish has surprisingly good Special Attack for when it starts the game and Gloom is still pretty alright, but if you're not playing Crystal the fact that he can only reach his final evolution at Kanto really murders him. Otherwise, his stats are middling.
Movepool: Pretty bad, to be honest. Grass/Poison isn't the worst typing, but offensively it's walled by Team Rocket, not a good thing in GSC. Even worse, his best level up move until
level 24 is
Absorb. What does he get at level 24? Acid. As Gloom, he does get access to Moonlight in the late 30s, but it takes him until level 44 to get Petal Dance. Kanto brings him Giga Drain, though. The Rockets do give him Sludge Bomb and early on he can utilize Cut, but both of these are coming off a sub par attack stat(at least Sludge Bomb gets STAB). He can also try Solarbeam on the way to the E4, as he can be taught Sunny Day from the radio tower.
Gyms that it fares well against:
This is probably where Oddish hits a wall. While he's not murdered by Team Rocket like other grass types, he's still a Grass type in Johto. Matchups:
Bugsy: He can sort of take down Metapod by Poisonpowder stalling him with Absorb, and maybe put the others to sleep, but that's all Oddish wants to be doing in this fight. Kakuna isn't poisonable and takes almost nothing from Absorb, as does Scyther, and Scyther can really hurt him with QA.
Miltank: Hm. Oddish can probably take down Clefairy without any problems as even if she mimics something it won't matter. Miltank will crush right over him, but he can paralyze her or put her to sleep to assist the rest of the team... Won't hurt her any, though.
Morty: Doesn't even matter if you're Gloom yet. Morty's gym utterly resists basically anything Oddish can toss at them, taking almost no damage from Absorb and being immune to cut. Status is helpful if you maybe want to slow something down, but I wouldn't leave it in on Gengar's Dream Eater anyway. Gloom should just stay far away from here.
Chuck: Gloom has a surprisingly good matchup here! Part poison means he resists fighting, and he can probably do well against Poliwrath because he resists both of its attacking moves.
Jasmine: He resists electricity, but he can't do anything back to the Magnemites. Iron Tail will kill him too.
Team Rocket: While he's not murdered by poison like the rest of the Johto Grass types, he can't actually do anything back except Cut. That works for a while, but it's not going to help against Koffing much. As a note, this is where he finally gets Sludge Bomb.
Pryce: Honestly, this is a weird matchup. Theoretically he can take down all the pokemon he uses with Grass STAB, but he's
still using Absorb, and thus probably won't even 2HKO anything. Gloom's SpDef isn't the best in the world (though it is nice) and Dewgong will probably get rid of him with Icy Wind. He might have a chance against Pilowine, I guess?
Clair: Unless he's overleveled and has Petal Dance, all he can do here is spam Sludge Bomb and maybe sleep something. Two of the Dragonairs ruin his day, and Kingdra is questionable due to Dragonbreath. Still, he can sleep her, so that's neat.
Will: Yeah, no. Nothing on this team doesn't have Psychic, so Gloom is really going to suffer. If you're playing Crystal, this is probably when you're going to want to use the Leaf Stone (Because of finally getting either Solarbeam and Sunny Day or just Petal Dance) but even he won't want to take Psychic from anything here, especially Jynx.
Koga: If you've got Moonlight, you might have a chance here. A set something like Sludge Bomb/Petal Dance/Moonlight/Cut offers decent enough coverage to wall Ariados, but be careful because Ariados could easily Spider Web you and pass it to Venomoth, who you really don't want to be trapped with. You can't kill Muk or Forretress at all and you don't want to go up against Crobat, so this is another meh matchup.
Bruno: Surprisingly good. Hitmontop can only hit you neutrally and if you have Moonlight you can abuse the turns he wastes on Dig, while all of Hitmonlee's moves are resisted. Onix dies in about half a second, although since he outspeeds he can possibly set up Sandstorm which makes Moonlight a lot less effective. Hitmonchan might be a minor problem with the elemental punches but Vileplume's tanky enough that they're nonissues. Machamp, however, is a major problem. Resisting Cross Chop doesn't matter because he likely can't be 2HKOd by Petal Dance and Vileplume doesn't like strong Rock Slides.
Karen: Eh. Umbreon's possibly too defensive to take down and can probably annoy Vileplume to death or stall her beyond sensible limit. Your Vileplume probably wins against hers due to SBomb, but it may take a while due to middling attack. Murkrow resists Petal Dance yet lacks flying moves and so is easy fodder for SBomb, but Gengar is able to take a few hits without trouble even if she's running the absolutely pathetic Lick. And I'm not even going to bother with Houndoom.
Lance: Hmm. Gyarados might lose to you but you really don't want to take that Hyper Beam. Rain Dance means Moonlight won't help much either, so you'll probably want to stay away. Two of the three Dragonites will murder you with their coverage move and the last one can still Hyper Beam you into oblivion, and obviously Charizard and Aerodactyl are both suicide.
Lt. Surge: Kanto is never really a problem. If you're playing GS, then you might actually have a Vileplume now. Anyway, Surge's team can't hurt you in any way except by exploding on you, so this is a cakewalk.
Sabrina: While Vileplume can deal decent damage, this is basically Will part 2 and so Vileplume need not attend.
Misty: It'd seem like this would be where Vileplume finally comes in handy, but it isn't. Everyone here except Quagsire carries a super effective move, so you're best used hopping in to KO Quaggy and then hopping back out. Somewhat obnoxiously, Quag can actually get an Amnesia in, but Petal Dance still KOs. Just watch out if you get locked in.
Erika: Tangela can put you to sleep but everyone except Victreebel is easily taken care of with Sludge Bomb. With Moonlight and Sludge Bomb, you can easily take down Victreebel before her Acid does.
Janine: Everything you have is resisted except Cut, and Crobat and Venomoth will probably both eliminate you.
Brock: Giga Drain/Solarbeam/Petal Dance destroys his entire team.
Blaine: Vileplume should sit this one out. I'm pretty sure nothing you have can get even a neutral hit on Magcargo.
Blue: Eh. Rhydon is naturally eliminated very quickly, and Exeggutor doesn't have any psychic moves and is thus vulnerable to Sludge Bomb. Pidgeot, Alakazam, and Arcanine are all bad news though. Dunno about Gyarados, but I wouldn't recommend fighting it.
Red: Hm. Pikachu can't kill you, I don't think. You might be able to take down Venusaur too, as he can only hit you for 4x resisted hits while you can hit back for neutral damage. Everything else either eliminates you or is simply too difficult to kill (Snorlax) so you should just leave them alone.
Other info: Vileplume's biggest two problems are severely bad damage early on and a difficult evolution. If you evolve it early in Crystal, Gloom doesn't get Petal Dance and has to subsist on Absorb as its best(Or should I say,
only) grass attack until Solarbeam, which is right before Victory Road. If you're willing to waste some time, you could try breeding Razor Leaf onto it, but the only ways to get it would be breeding from the Sprout family (who learn it at level 37 and by then you should just use the sprouts instead) or from Chikorita(and if you have Chikorita you shouldn't be using Oddish anyway). These mean that essentially it cannot be rectified until the E4, so using it's honestly an uphill battle. Cut being your best move against most things is very unpleasant, and being walled is never fun. In the end it's not so bad that it can't be used, it's just terrible enough that almost any other Grass type probably does its job better. The poison neutrality is pretty good, though, and it's fairly usable in Kanto.
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Once again, sorry about the excessive length!