Alright, so I have been playing through Pokemon Yellow for the last few days so that I can see how well Charmander and Squirtle fair in that game. I haven't beaten it yet, but I just defeated Giovanni for the last (and most disappointing) time. So, here's what I've got. (Keep in mind, I've also been using a Pikachu, a Nidoqueen, and a Snorlax. Probably should've been using at least one fewer.)
Yellow Charmander
By the time I got the Lv 10 Charmander, my Pikachu and Nidoqueen were both around Level 20 (give or take a couple of levels). I probably babied the Charmander a little bit, feeding it roasted Oddish and Venonat and switching him in and out. However, I ended up with a Charmeleon one battle before I reached Bill's house. Ember was okay for picking off the plants and bugs he faced along the way, but Charmeleon's greater weapon was Dig, allowing me to knock down the Fire, Poison, and Electric Pokemon that stood before us. He could have 2HKOd Surge's Raichu with Dig (would that be a 4HKO technically if the move takes two turns to use? :/ ), but he missed one of those attacks and then later ate a critical Mega Kick.
Thanks to the Celadon Gym and Pokemon Tower, Charmeleon gained a ton of experience pretty damn fast, and I had a Charizard after beating Jessie & James at Pokemon Tower. Dig has since been really good for the foes I mentioned before, but Slash has actually been impressive, too. Ember was looking pathetic by comparison. He managed to take down Koga's Venonats using Fly, but his Venomoth was a bastard and slayed Charizard (as well as everyone else besides Wartortle, who was lucky enough to end that psychotic moth who was barely hanging on).
After that, against most regular trainers, Slash and Dig (with occasional flight) managed to get Charizard past most foes he fought. For a long time, he was the highest-level Pokemon on my team by the end of the mid-game. Even Water-types aren't really scary with Slash. I've been liking him quite a bit, though I have seen times when he's a little shaky, like when Giovanni's Nidoking fried him with Thunder. I'm thinking around High-tier for Yellow Charmander. I guess the ultimate point is that in both cases (RB or Yellow), Charmander arrives at a point where he can level up fast and has enough at his disposal to shred a lot of the game. The version doesn't seem to matter much here.
Yellow Squirtle
Squirtle also comes at Level 10 when the wild Pokemon around Vermilion are Levels 15-18 or so, as well as when my three other monsters (Pikachu, Nidoqueen, and Charmeleon) were Levels 25-27. My first thought (besides forcing that Bubblebeam TM down her shell because Bubble blows) was to train in Diglett's Cave. First Diglett was at Level 19 and knocked out Squirtle. Yeah... So, I decided to grind her against the other half of the trainers I didn't already beat before the S.S. Anne. The only Pokemon that Squirtle beat on her own was a Gambler's Vulpix. For the others, I switched her around. And she wasn't even a Wartortle by the end of Route 11. She did gain a little more experience on Route 9, however, since that route had more Hikers and did evolve then.
Since then, I used Wartortle rather conservatively, mainly saving her for combat against opponents I knew would have Ground Pokemon. She did see some use in Celadon Gym, but that was only because I taught her Ice Beam. She would also pop out to fight trainers with birds sometimes, but I also had Pikachu with Thunderbolt and Nidoqueen with Rock Slide, so that wasn't really a big deal. Until I beat Koga and Sabrina, Blastoise was pretty much my lowest-level Pokemon. She did, however, warrant a lot more usage with the last two Gyms. Maybe Blastoise could've been more useful if I taught Earthquake to her instead of Nidoqueen.
So, in short, Water Pokemon don't seem to be that useful in the middle of the adventure. Blastoise was great against certain foes but not that strong against others. Maybe she would've worked better with some better attacks (or without Nidoqueen around), but Blastoise was so-so. I'd support Mid-tier for Yellow Squirtle. Not much higher but no lower either.
