Quote: "is there a poly limit on models for FPS's?"
Yes but it depends on the machine trying to run it. A good guide is to try to use LOD(Level Of Detail), so you have several poly count versions of the same model which get used at different distances from the camera (there are free plugins you can find on the forums that can help with switching between your low/high poly models).
Quote: "I am thinking no more than 30 characters on a screen at once and average 5-6 most of the time."
I would try a few tests of your character models and see how it performs. You can hide models that aren't in view too.
Quote: "What about enviorments? I need a very rich outdoor enviorment to pull this off. Suggestions?"
Blitz Terrain looks like a fast and elegant solution for terrains but you'll need to think about plant/trees too. PlantLife and TreeMagik are relatively cheap ways of filling terrain.
Start simple and work your way up. Get a terrain working (start with advanced terrains using native DBP commands) and then get a character moving around on the terrain and go from there...
That's my advice anyway
