Actually thats what I found out!!
The power generators are always at the same spot on the same map, so do the ion-cannon/HV-gun(they share same spot) and the several spots for turret, and also the shield generators. The bases for native species are always the same. Places to buy upgrades, and other green/neutral buildings are always the same.
As for the rest of the buildings, there are a few "slots" on a map gpr them, though which is which tends to be slightly random.
Oh, for predesigned skirmish map, everything is set with no variation, so technically a map should exist.