Shellder probably deserves another look, it actually seems pretty good. Can't go wrong with Water/Ice lategame, especially if you throw in Ice Beam or Blizzard TM. The fact that it doesn't even need those to have Ice STAB is really good too.
I was sceptical on Tauros as well and would not mind putting it to Low. I was kind of convinced to put it to Mid when I realized it doesn't _need_ any TMs (as opposed to what I first thought), though without any high-want TMs the best move it has is Strength.
I don't know why Farfetch'd is still in Low. I rallied for it to be put in Mid so much but then when I got control of the list I never actually moved it. I actually used it in-game so I'll rewrite it.
From my experience you generally get Bulbasaur later and lower than Bellsprout when all is said and done in Yellow unless you use the Potion glitch (which we outlawed). So that could be the reason. Bellsprout also starts out with a Grass move that lets it take out Geodudes and Onixes, whereas Bulbasaur is stuck Tackling for a bit. I agree with your reasonings though, and both should probably be in the same tier.