Im willing to listen more to Jolteon and Flareon but... waiting until Level 30 as an Eevee is pretty bleh. At best you can go with Return STAB and Headbutt. Even then Eevee is kind of meh as a combatant. Ill look into it but I think if the trainer is seriously considering Jolteon and Flareon going to the Game Corner ASAP seems mandatory.
Teddiursa also waits until lv. 30 to evolve, but he does have 80 base strength (same as Miltank haha) and Dig access.
This mostly only applies to Jolteon - Flareon can evolve right away and just hit stuff with physical moves (his coverage isn't that great however).
Vaporeon and Staryu can rise up (C anyway). Ill agree to those.
Why only C? They seem quite comparable to Golduck, and have better availability than Suicune in the same tier (one of the really questionable A-tier residents IMHO).
Im thinking Im going to raise Mareep to A. Being the best Electric is a bit more rewarding than being the best Grass-type. This is undermiming that Ampharos has Fire Punch at its disposal and, though meh in Speed and typing resists, it still has good durability to back it up. Nevermind Electric actually is useful offensively throughout the game where Chikorita has really crummy points. Like okay I'll give Grass-type is actually nice against Water-, Rock-, and Ground-types but let's think about this from a realistic standpoint. There are a lot of Pokemon (Flying-, Poison-, Bug-, Steel-, and Grass-types). A lot of these - Poison in particular, are shockingly common throughout the game. I mean at least being the best Electric-type has some meaning in here. Being the best Grass-type is being first in the Special Olympics race.
I think we needn't focus so intensely on who's the best Pokemon of a particular type. Imagine Celebi were playable (okay, his in-game moveset would kinda suck but still), becoming the game's best grass-type in Meganium's place. In Diamond/Pearl, Infernape is the best fire-type, but it's as good as it is because of the qualities it possesses not because it beats Ponyta in the pointless contest for the game's best fire-type.
So we should ask again, is Ampharos really good enough for A-tier and is letting it into the tier lowering the standards for possible future candidates? Because a lot of other Bs would probably want the same position due to not being much (if at all) worse.
And wrt the usefulness of the electric typing offensively - well, so many things learn Thunderpunch, don't they? Typhlosion, Magmar, Alakazam, Gengar, Nidos, Ursaring... You also have Spark Lanturn and Lapras who could always take the Thunder TM for the Rain Dance combo. All of them are faster than Ampharos and bring other appealing qualities to the game.
Steelix, IIRC, still has some credibility here. Onix is about C at the very best here. Onix without trade is good up until about Morty where it sort of falls off into nonexistence. Its offense is crap, its durability gets questionable as the game progresses, and the only real "setup move" it has to help itself is Screech (which has so-so accuracy on top of it).
It might still kill some things as quickly as Geodude due to being way faster. Outspeeding Gastlies in Morty's gym is one of the examples of this utility (and then you just one-shot them with Dig). And you just trade-evolve the thing when it starts getting weak in comparison to your other mons. It's likely your best steel-type in this game, and being a steel-type has never been bad.
Any particular reason why you (and atsync as well I guess) left out the Nidos out of A tier? I agree their place isn't cemented there but it could probably use some discussion.
Much like Lucchini mentioned I think Sudowoodo in C is kind of gimping it. Sudowoodo does have some nasty downfalls but it actually has a great movepool to compensate (Low Kick, Rock Slide, Dig / Earthquake via TMs). Though it comes later than Geodude and Onix it probably stands a little better in midgame than the two and then becomes mildly worse late-game.
Sudowoodo and Graveler are about even in the lategame I'd say. Sudowoodo has the better typing and actually a strong rock-type move (because relying on Rock Slide sucks).
I think the potential issue is B starts to pile up but I think that's the worst so far. We can discuss others like Slowpoke dropping to C if Psyduck goes to B (and maybe some C mons to D).
Slowbro/king have Surf, Ice Punch and Confusion for fighting-types so I don't see a problem with keeping them in B. Unlike Golem and Ampharos though, you don't have much hope for them outspeeding even the slowest things, but there's always the Quick Claw.
Overall, I find the lack of water-types in A-tier a bit bizarre considering how Surf's availability gives them a huge advantage over any other typing in the game (normal-types need the Friend Ball to achieve a similar neutral damage output). Crystal Starmie, Crystal Vap and Golduck should all be in A, maybe Lapras as well because of natural Ice Beam. The A-waters that nobody seems to have any problem with being there (Wooper, Suicune) are the really questionable ones IMHO.
And I wouldn't worry about some tiers being "too small" or "too big". As long as the Pokemon inside are sorted correctly, that's just the way things really are.