@DeepImpact: Even a pokemon as stupid as Slowpoke can discharge a deadly flamethrower or use powerful psychic abilities and they're slower than most zombies I've seen. Pun intended.
But seriously, if you make an RP where Pokemon might as well not be pokemon you're only reaaally appealing to zombie fans. That's not a bad thing per se, but in a world where some RPs struggle to survive you may want to avoid narrowing down your audience like that.
Better yet, why not have pokemon on both sides? It worked for humans against human zombies. They're much more numerous and may very well be stronger but you're smarter and etcetera.
Oh, did I mention that we've totally had pokemon vs human roleplays? I think I saw something recently about Nideous planning on remaking one of them, called The Children are Haunted. It had a pokemon vs humans theme, with some pokemon and some humans more neutral than others.
There was also another RP I think, that had a more clear two sides deal. I believe they gave humans themselves the ability to wield pokemon powers to even the playing field. Neither of them made pokemon powerless mooks and they both made out very well as I recall.