When I first put my new chicks in the coop, I put in a box with a small hole cutout which the younguns could fit through, but the mature hens couldn't, that way the little ones could escape if they were being picked on too much.  It worked pretty well.  I'm pretty sure it was in the coop for less than a month and maybe just over a week. 

 Oh thats a good idea, thanks for that.