My Poor Pullets

O.K., I just got 2 EE---I think----pullets last weekend. I already had 21 chickens, 4 of them roos and 7 of the hens are banties, while the remaining hens are full-size. All these previous chickens get along quite well. 5 were purchased last year as chicks, 8 this spring as chicks, and the 7 banties th is summer as 1 yr.-olds. Now I get thes e pullets, and I can't seem to integrate them into the flock----they keep getting picked on. I have tried pretty much everything-----keeping them in with just the banties during the day (as they are the lowest in pecking order); keeping them by themselves during the day, and in a cage in the coop at night so the others can be around them but not hurt them; and waiting until everyone else is roosting in the evening, and then putting them in the coop loose. Nothing seems to be working-----the one pullet has a "bare" butt, and the hens will peck until they draw blood if given a chance. How long will I have to keep these girls in a cage in the coop before this stops? Also, could I cover the one pullets butt in some way to protect it? I've really never had problems integrating before, so this is frustrating. Any suggestions are welcome.