700 polygons per object is maybe too much for big amounts on screen. It is also possible to make extra-low poly models or like mentioned before, simple sprites for objects in distance.

How are you going to handle 10000 ships combat in multiplayer? In event like that rendering is only like 10-30% of processing power (Curious because i'm making very similar game myself)

How many players should be able to attend same time with their armies?