Server performance and polygon counts have nothing to do with one another. The only ways the server can be affected by a high polygon count are:
A) A complex SUR (hitbox), which would render hit calculations more complex and thus slower
B) The client starting to have graphic lag, which would cause delays when sending information to the server and thus cause lag for that specific player
However, any modern computer won't encounter B and A can be avoided with good design (a SUR can be as detailed or as simple as you want with no relation to the model's own detail level).
As for a hard limit, there is none. The limit is your computer's (and those who will use your model(s)) performance. Freeworlds: Tides of War, for instance, has a 100,000 polygons Imperial Star Destroyer and there is no significant lag. Likewise, 2048x2048 textures were used in Freelancer: Combat Evolved quite extensively with no perceivable performance hit. The biggest issue with large textures actually isn't lag, it's memory limits. The better your GPU, the higher resolution textures it will be able to show. Old mobile GPUs would display a blank texture when a 2048^2 texture was used.
Everything depends on the kind of machine your target audience has and how many of the models you build will be shown on screen. 10 models at 100,000 polygons would be impossible, but if it's just one or two it'd work. Of course, if you're specifically thinking about submitting to Discovery, you have to abide to their predetermined limits, but otherwise you have quite a bit of freedom overall. Just remember that a lot of polygonal details can be rendered just as well with some texture trickery.
Finally, I'm afraid FL doesn't support many advanced techniques. You have glow maps and that's about it. Glow maps are just textures where black is transparent and where whatever is show is always visible, even in pitch darkness.
