There is not really a point in having the combat engineer all it does is increase construction capabilities with almost no stats and taking a lot of training time. The only reason you would use it would be to construct a PDC. You can also use the ships with drop capabilities to board enemy craft and take the vessel.
You are mistaken. The
Construction Brigade is the one that does construction like a factory, and is truly huge in size requirements. The
Combat Engineers are units that get a bonus to combat from and against PDCs.