To be safe I wouldn't put any in the same brooder that are more than 3 weeks apart. I have a small 3 week old in with some bigger 4-5 week olds, and they barely seem to notice that she's smaller. In fact they seem to protect her like an older sister! However i did not put my new 4-5 day old babies in with the older girls, even though I stuck 3 week old Caliope in with the babies and they seemed fine. But her older sisters missed her so i put Caliope back in with the big girls. So if you are committed to watching them closely it could be fine putting the little ones with the older ones.
