I think part of the issue with a large pokegirls game is that it's very reliant on someone not leaving, since by necessity there's several groups of two or more PCs that are closely related to each other, and having one of those PCs' players leave means you have to find a replacement to continue.  You could have each player play as both tamer and harem, but that would mean all the taming scenes would be written individually, which (in my opinion, at least) kind of defeats the purpose.