Carvanha
Availability: (Mid) You can catch him at Route 22, but in order to do that, you have to get the Good Rod, which is obtained at Couramine City, so you'd either have to do significant backtracking in order to go catch him, or progress a bit more through the game and miss some exp opportunities to basically take the easy way out, though this review will assume one does the prior. He'll be obtained at level 25, pretty low considering that the next gym has Pokemon in their mid-30s. However, he evolves at level 30, so he isn't dead weight for too long.
Typing: The Carvanha family is Water/Dark type, which will be a curse for the first three gyms he can participate in, but has no particular disadvantage against the elite four. Offensively, Water/Dark coverage is pretty good. Being walled by few opponents, though some of those few are used in major battles.
Stats: 45 / 90 / 20 / 65 / 20 / 65
Sharpedo: 70 / 120 / 40 / 95 / 40 / 95
Sharpedo is terrible at taking hits, and Carvanha living a neutral hit is rather miraculous, so you need to baby him until he at least evolves. His speed, while not the greatest, is still pretty good, and his mixed attacking prowess shouldn't go unnoticed.
Movepool: (Average) The second you catch Carvanha, his only attacking move is Ice Fang, which is decent for coverage, and you can teach him Thief as a decent physical STAB attack, with Surf as his best special STAB (Hydro Pump is bred). At level 28, he learns Crunch as his most powerful physical STAB, being tied with Waterfall in terms of power. TMs include Bulldoze at Lumoise City for Electric-types, Rock Tomb at Cyllage City for Bug-types, Poison Jab at Shalour City for Fairy-types, Ice Beam at Snowbelle City for Grass-types, and Earthquake at Route 21 to replace Bulldoze, though the first two become redundant late in the game, Poison Jab is kind of bad when not hitting Grass- or Fairy-types, and Ice Beam is obtained too late in the game, as well as the fact that the prior three can only be taught to Sharpedo.
Major Battles:
All Rival Battles: You can take out Meowstic, Altaria, Flareon, and the Fire starter, and do adequately enough against Absol and Vaporeon, but the rest are bad matchups.
Team Flare: Smashing through their Fire-types is easy work, and resisting Dark means you can actually live a few hits. Though neutral matchups are where things go south, such as when against Poison-types.
Ramos: You don't stand a chance here except killing Jumpluff with Ice Fang. Sharpedo's deadweight here.
Clemont: Bulldoze can nail the frail Electric-types, but if anything can take a hit, you're dead meat.
Valerie: Poison Jab the frail Fairies, stay away from anything involving bulk. He does surprisingly well here, but gets crushed by any Fairy-rype move.
Olympia: Finally, a gym Sharpedo can destroy. His high attack and speed combined with Crunch alone can take care of near anything in this gym aside from a Medicham.
Lysandre: Pyroar is vaporized and Murkrow/Honchkrow won't be a particular issue, but Mienfoo/Mienshao can easily KO you if you don't get the kill first turn, and (Mega) Gyarados can literally tank Sharpedo and go for the kill.
Wulfric: Aside from Abomasnow and Avalugg, no real disadvantage, and Ice resistance helps.
Wikstrom: If you got Earthquake, you can take out Klefki and can reliably OHKO Probopass (2HKO with Sturdy intact), and Crunch can hurt Aegislash, but might not KO while he and Scizor can KO you in return.
Malva: She has no chance at winning against you. Her Torkoal can possibly live a Waterfall and retaliate with Earthquake, and her Talonflame could outspeed Sharpedo, not to mention the risk of a Flame Body burn, but that's literally it.
Drasna: Ice Beam really helps against Noivern and Altaria, and Earthquake can take care of Dragalge, but Druddigon can live a hit and retaliate.
Siebold: Half advantage, half disadvantage for this battle. Crunch takes out Starmie and Earthquake can wound if not KO Barbaracle, but the rest can live a hit and take you out.
Diantha: Hawlucha lead destroys you, everything is too bulky to KO without hitting them super-effectively. The only reliable matchups are Gourgeist and Aurorus, while Tyrantrum and Goodra can live a hit and KO with an attack of choice. Mega Gardevoir takes a lot of damage from Poison Jab, but this is a shaky KO, while Mega Gardevoir can easily kill with Moonblast.
AZ: All three are weak to STAB, so this should be fairly easy.
Comments: Sharpedo isn't exactly a very good Pokemon for an in game run, but he's certainly not the worst. He has several bat matchups, but does have his moments. His defenses are pathetic, but he has great offensive potential. He's a so-so Pokemon, and that's just fine with me. D or E rank.
Ferroseed
Availability: Glittering Cave (Early) Ferroseed can be obtained just before the fight with Grant, which is a very convenient time to catch him. He evolves at level 40, so around the time you get to fight Valerie.
Typing: The Ferroseed family is Grass/Steel type, granting many great resistances at the cost of very few weaknesses. These weaknesses are bad news when facing certain battles, but resisting most of the game's attacks is amazing. Offensively, Grass/Steel coverage is walled by a few types, but does well enough for the most part.
Stats: 44 / 50 / 91 / 24 / 86 / 10
Ferrothorn: 74 / 94 / 131 / 54 / 116 / 20
Tanking hits is what the Ferroseed family does best, but they can hit back well enough. Their terrible speed is what turns many players away from using him, but that should only apply to a speed-run.
Movepool: (Below Average) Upon capture, Ferroseed has Curse and Metal Claw, already making him a lethal set up tank. He also comes with Rollout and he gets Pin Missile at level 18 as some form of coverage, but they're really just there for when something resists Steel. At 21, Gyro Ball is leveled up, being his strongest move in the future, and can work wonders with Curse, but it's not going to be at 150 power since the speed differences are less significant at low levels, but it should still do good damage. At level 40, he gets Power Whip, which will finally allow him to take on Water-types more effectively. For TMs, the only notable moves Ferroseed can use are Rock Smash at Ambrette Town and Poison Jab at Shalour City, but Ferrothorn gains access to Bulldoze at Lumoise City store, Aerial Ace at Connecting Cave, and Shadow Claw at Glittering Cave. A notable move that isn't offensive is Thunder Wave at Route 10, so more easily capturing Pokemon/crippling opponents is good to have. If needed, Ferrothorn gets Cut and Strength.
Major Battles:
Team Flare: Fire-types and Scrafty are bad news, but they're really the only concern.
Grant: Ferroseed can set up on everything and proceed to KO. Nothing stops this thing once it starts setting up.
Korrina: This is one of the few bad matchups Ferroseed will ever face. Fighting-types prey upon him, so he's no good here.
All Rival Battles: Meowstic is set up bait, Fire starter and Flareon can easily KO, and everything else is taken care of with no problem.
Ramos: You resist most things thrown at you, and you can set up on everything.
Clemont: I'm getting tired of saying this, but I'll say it anyways; easy set up and sweep.
Valerie: You can kill everything here, even without setting up.
Olympia: Aside from a Medicham, there shouldn't be an issue here.
Lysandre: Mienfoo/Mienshao can take you down, Pyroar requires no effort to OHKO you, Murkrow/Honchkrow aren't too bothersome, and (Mega) Gyarados is easily taken down.
Wulfric: Another easy time without the need to set up.
Wikstrom: You can set up on his Klefki and sweep from there. Aegislash can hit you with Sacred Sword and bypass defense buffs, but that's really it.
Malva: Ferrothorn won't last a second here.
Drasna: Other than her Noivern with Flamethrower, there won't be any problems here.
Siebold: Clawitzer can nail you hard with Aura Sphere, but defeat him, and the match is as good as yours.
Diantha: Even though Hawlucha can't 2HKO you, she has Swords Dance to counter Curse, and Goodra can easily kill with Fire Blast. The rest shouldn't present a problem.
AZ: Torkoal is the only thing you can't take on. The other two are an easy time.
Comments: Ferrothorn is an amazing Pokemon to use, the only flaws here are lacking some good coverage until he evolves and speed, which really ruins speed-runs. Other than those, he's an amazing Pokemon to use. As long as you're not doing a speed-run, put him in A rank.
Skiddo
Availability:
Route 5 (Early game) Skiddo is available just after your first visit in Lumoise City, which is a convenient time not only because of getting good TMs then, but also because Grant is the first gym leader he can battle.
Typing:
The Skiddo family is Grass type, which is a bit difficult to manage, considering its multitude of weaknesses that hinder you in a few major battles, but also carries helpful resistances which help against several other major battles.
Stats:
66 / 65 / 48 / 62 / 57 / 52
Gogoat: 123 / 100 / 62 / 97 / 81 / 68
Gogoat is pretty good at taking hits while hitting back hard with either offensive stat or both of them, but his low speed can get in the way of a sweep, and his physical defense isn't exactly the best.
Movepool:
(Average) Skiddo starts off with Vine Whip, which is a decent move to throw around for a bit. At level 13, he can replace Vine Whip with Razor Leaf, and at level 30, he can replace that with Seed Bomb, and at level 55 as Gogoat, Lead Blade replaces that. At level 22, he learns Take Down, which helps against Flying-, Bug-, and opposing Grass-types. At level 34, Gogoat learns Bulk Up, which will be a good set up move for sweeping. For TMs, Bulldoze is available right before you can catch Skiddo, so he already gets a good coverage move for the time. He can replace that with Earthquake either at Route 22 or when he hits level 60 as Gogoat, whichever comes first. Aerial Ace found at Connecting Cave has its moments, but is rather situational late game. He can get Surf from Serena after you beat Korrina for the second time, which can help against frail Fire-types, and is good to have for travel. He also gets Strength if you need it. At Couriway Town, you can find Rock Slide, which helps against Flying-, Bug-, and Ice-types.
Major Battles:
Team Flare: Zubat/Golbat and Fire-types are the only things that specifically threaten Skiddo/Gogoat. Everything else is fairly easy to plow through.
Grant: This is just way too easy. Amaura and a trainer's Dwebble should be the only things that can take Skiddo on. The rest are easy pickings.
All Rival Battles: Your rival's Fire starter will easily KO, and other than Vaporeon with Aurora Beam, nothing else can threaten you, especially if you have a few Bulk Ups under the belt.
Korrina: If you have Bulk Up now, this gym can only threaten you with Hawlucha and a trainer's Heracross, but is a rather bat match up without it, even if you have Aerial Ace.
Ramos: You surely have Bulk Up by now, and if you also taught Gogoat Aerial Ace, this gym is a cakewalk.
Clemont: You resist Electric-type moves, have Bulldoze to throw around, and Bulk Up to ensure one-shots. GG, Clemont.
Valerie: No real advantage in terms of typing, though a move or two can hit some things super effectively. Mostly a neutral match-up.
Olympia: Sigilyph is a pain to deal with, but the gym is mostly another neutral match-up, otherwise.
Lysandre: Honchkrow/Murkrow and Pyroar can threaten Gogoat. Mienshao hits on his weaker defense, but he can live a hit and retaliate. (Mega) Gyarados is a good match-up for you.
Wulfric: If you're crazy enough to use Gogoat here, he can hit things with Rock Slide, but one hit from an Ice-type move and he's down.
Wikstrom: Earthquake takes care of everything here except Scizor, who can nail you with X-Scissor, but a few Bulk Ups remedy this weakness.
Malva: You can hit everything here super-effectively with Rock Slide or Surf, though everything can rip you a new one with their Fire-type moves.
Drasna: Dragalge and Noivern can threaten you with Sludge Bomb and Air Slash respectively, but the rest are small fry.
Siebold: Gyarados has Ice Fang, but that's the only thing that you're weak to. The rest is smooth sailing.
Diantha: Hawlucha lead takes you out, Goodra can use Fire Blast, and Mega Gardevoir can muscle her way through your defenses. The rest are nothing too threatening.
AZ: Sigilyph beats you, Torkoal goes either way without Surf, and Golurk is beaten.
Additional Comments:
Gogoat is by no means a bad Pokemon, he's just outclassed by Mega Venusaur as a tank, Ferrothorn as a set up sweeper, and Simisage as an attacker, but finds himself as the middle road between the three, which can be reason enough to use him. He's a jack-of-all-trades kind of guy, but he's also master of none. I'd say put him in C rank.