The MeshX program can reduce the poly counts of your Doga ships. 3d Studio Max can also do this as well, but for their .3ds format before you'd convert it to a .x file.
The stock ships are viewable and can be modified in a program that can work with .x, and very nicely done as you can see in most screenshots.
There are fewer ship sizes than se4, but the size ratio is about the same between the ones that are in the game.
That's about as much detail I can say here...