I had no problems integrating with a 10 day difference. If you can put the babies in with the older girls with a wire divider between them for a day or 2, (obviously, give them their own heat, feed and water sources) then you should be able to take it down without any problem. I've heard that integration works well up to a 3 - 4 week difference. As long as the little ones are past the baby "can't walk without falling down" stage.
how big is your brooder, and how many chicks? How many bantams, How many large fowl? Are they picking to the point that the banty's can't get away, or just picking because they see the toes. Are they drawing blood? There are a few products that you can paint on the pickees to keep the pickers from picking. If the brooder is not big enough that they can get away from each other, you'll probably have problems. Do the bantys have feathered feet?
Just read back through your post. The older girls are leg horns, right? From my experience with that breed, they tend to be more flighty and agressive. I'd just keep an eye on the situation, and intervene if it's getting really nasty. You might try taking the leg horns out of their "home area" for a few days, or you might try separating them again, then putting a single leg horn in with the younger chicks, best to do it when you're offering a tasty treat. (Choose the least dominant one) If that goes well, add an other leg horn... etc. If that doesn't work, you've really got a struggle on your hands. But don't give up... hopefully they'll settle into a pecking order, and the agression will diminish as long as every one remembers where they belong in the pecking order.
