Obviously the lower the poly count of your models the better, but not so ridiculously low that they look boxy, and stupid in game. A good modeler can make a mesh as detailed as possible with the lowest poly count possible. You dont need a 10,000 triangle model when 2000 triangles along with good texturing, and bump mapping will look exactly the same (or in some cases BETTER) in game. Use the LOWEST poly count possible to portray your art.
You will also have to take into consideration how many of your models will be on the map at the same time. If only a few will be on the map you have more room to play with poly wise. On the other hand if thousands of models will be on the map then you need to budget the polys accordingly.
Strike Craft should not be higher than about 700 triangles. Trinity strike craft vary between 500 to 700 tri's
Frigates in Trinity average 1500-2500 triangles. Though again in Trinity there are some frigates in the 3000's. The vasari trade ship is in the 3000's. Which is why we reduced the poly count for trade ships, and constructors in TSOP. 3000 is TOO HIGH for a non combat NPC that spawns in the hundreds.
Trinity Cap ships average between 3000-5000 tri's (trinity meshes). Which is reasonable considering there is a limit to how many cap ships there are in game. I am assuming that the new Rebellion cap ships are similar in poly counts.
The highest poly count ive seen for a Structure is 7000 tri's (tec hangar defense). Considering how the AI spams structures i would go MUCH lower if possible.
10000 for super units like starbases, and novaliths. A fully upgraded Advent starbase is over 13,000 tri's. Again i would go much lower if possible, because of the AI spam.
The Titans i have no clue of their poly counts yet. But from what i see in game the number looks big.
Corvettes i would say dont go higher than 1500, but i dont know the actual corvette poly count yet.
I will have to re-import every mesh into 3dsmax again to get an accurate count. TBH i dont want to do that.