Well, if it was me, i would REALLY not want to pass up the free Auracanas, but I would be afraid to try it. Shutting them up after they are used to free-ranging would make the situation even worse, I am sure, as they will be bored. If they ever peck one of the little ones and draw blood, the rest (even the little ones "siblings"), will peck at the blood till they have killed the little one, unless you happen to find it in time.
Maybe someone else will put in their experiences here, and maybe I am over-cautious. this is just my opinion. Good luck with whichever way you go.
One possibility, if where you are putting them, there are several places they can crawl into, or behind, to get away from the big chickens, this would help, but on the other hand, having to stay crawled behind something, and probably being afraid to come out and get food and water, wouldn't be much of a life for the young ones.