[commonbullet]

Custom models - I'm not sure what's the limit. I know it depends on file size and how many polygons it has got to be rendered and how many stuff you are already precaching (including those maps with too many models, etc)

About the polygons, a custom model should respect the avarage poly count for a player model, so the game won't slow down.
I guess the worst part is precaching. I suppose the engine has a limited size for precaching files, and users has got a limited patient to download them.

I'll try to find more precise numbers. Or maybe someone in here has got more precise information.