So, I've just found a very very nice planet with some very very nice special buildings, including a sillix starport and human training barracks. The description for these two arts says that they can only be built by a sillix and a human. Does that mean that even if I extract those buildings, I will not be able to construct them on other planets without changing my race to sillix or human?

correct.... to place them you have to be that race at the time....