If these are the Karskins you mean then unfortunately i'm pretty sure they where only made in metal. The head, body, gun, and legs are all one piece, i think the back pack is the only piece that comes separate. While not impossible you would have to get the complete model and cut the heads off. While it would probably look really good if done right IMO it would be kinda a waste as I feel Karskin models are pretty awesome as they are and since they are out of print will only get harder and harder to come by.
The Scions might be a good idea as I believe the kit comes with a bunch of extra heads. I think its a good mix of the beret and helmeted heads but if you are only interested in one type then it might not be such a great idea. Tho you could try trading the ones you don't like with someone for the ones you do.