Bridges are mostly bogged down with textures.
The polygon figure sounds ok.
Regarding characters, I wouldn't suggest going over the EF2 character polygon counts.
Also remember, you need to add the character polygon counts to the total bridge polygon count.
Your best bet is to ask, specific, questions here.
Generally it goes like this:
You model the bridge, you texture it, you generate, and apply, lightmaps for it (ask when you get there).
Then you need to animate the bridge using the biped skeleton included in the SDK.
And then of course some scripting, I can help you there when needed (I've seen enough bridge scripts 

).
Also, the animations needed can generally be "extracted" (the types of animations, not the animations themselves) from the other bridges (look in the PreloadAnimations function/def of each bridge file, in scripts/Bridge, and the bridge plugin file of a bridge, found in scripts/Custom/Autoload/, it's the 'locations': { part).
As for the characters themselves, it's all an "uniform". ;)
But you must remember, only with BPUniforms can you get dynamic uniform switching.