You don't tell us how much space ten chickens have. If they're confined to a run smaller than 100 square feet, then that may be one of the main causes of the feather-picking.
However, feather-picking isn't a simple problem and it has no cut and dried solutions. A picking problem may start because of a dietary deficiency or over-crowding, but even after it's remedied, the problem may continue because it has become a bad habit. Trying to break a chicken of any habit, is a challenge for the record books.
I recommend you do what Judy has suggested. Then, when you put the little bullies back with the flock, sit there for a while and observe carefully. The second you see one of them going for a butt feather, holler as loud as you can, "NO!"
You should see the culprit freeze, and as soon as the shock wears off, go back to the butt she was about to pick. Holler, "NO!" again, and keep doing it until she wanders off to do something else. If she ignores you when you holler, get right down close and holler in her ear. That will do it.
Over the next few weeks, try to spend some time in the run, and if you see picking, repeat this holler "therapy". I'm discovering some surprising results from doing this with my own flock, which has several serious pickers, one of whom has been driving me nuts for five years. She's pretty much stopped her picking.
Good luck! Hope something works!