I think I'd go for nearer what henford said as at any one point there isn't going to be a lrage number of buildings except for power sations, supply centers, and base defences. SO all the other buildings can a have relatively high amount of polys. IMO anyway .
I think that sounds about right and henford who has made tons of bildings and stuff says upto 3000 he's probably right. Although don't tell him I said that.
his superweapon building he made was 4500 poly. also i think 2000 poly is a reasonable poly limit (3000 if you have alot of stuff like a shipyard that has alot of boxes and stuff).