About Poliwhirl... first thing: I LOVE POLIWHIRL, is one of my favorite Pokémon ever, too bad it doesn't evolve too well (I'm not particulary a fan of Poliwrath, and I dislike Politoed). Poliwhirl's stats before evolving are from being bad, actually, he's really fast, so, nothing will outspeed your Poliwhirl at all, it does have one of highest bulkies of NFE Pokémon, so, it can take a hit or two, and it's attack is usable, and it does have STAB Water Gun and later Surf (and you can still waste some TMs on him), and he does learn Body Slam! With Amnesia he can pretty much solo anything later on the game, except maybe Agata and it's status madness.
If you're gonna use Amnesia, there is not too much point in evolve Poliwhirl, anyway, Poliwrath's new weakness are kinda nasty, you're losing 20 base speed, and the difference of physical bulky is not excesive high to compensate the speed drop.
Amnesia = Stay as Poliwhirl
Not Amnesia = Fast Poliwrath
Don't forget he learns Hypnosis, for your in-game catchs.
I love Caterpie, he's a so adorable little bug! ♥ (Also, sorry for my English, I'm still learning it and I'm not a native speaker).