Limits of customizing content

[Bweetza]

I've seen some threads about modifying textures, such as this thing that I don't pretend to understand. Here's one mentioning modifying the client side weapon models. Are there any guidelines for what's permissible on P99? There are obvious cheats like making transparencies where there should not be, so that's off limits, but I don't want to run afoul of anything before I start trying to snazz things up.