Ships do add up to twenty I think, just as a total not as a per side thing.
Rebel space forces are:
CR90 Correllian Corvette
MC80 Mon Calamari Cruiser
Nine units in the demo, you also gain access to the A-Wing with the hacks, so ten for Rebels.
Imperials will be having:
Victory-class Star Destroyer
Imperial-class Star Destroyer
Tartan Patrol Cruiser
IPV 1 Imperial Patrol Vessel
Ten Imp ships. Fairly balanced group, but while neither side has 20 types, combined they add up to that total.
As for the build spots... done well they can work, to be honest though, I prefer Birth of the Federation's five resource building types which needed human resources, then a series of other buildings which had limits per system or per empire. Just works better for me, but then I probably see things that way because of some of the crud Rebellion used to pull by killing not only my buildings but my already limited amount of building slots when certain events went down.