We introduced four 2-4 day old babies into a brooder with three 3 week olds and one 2 week old chicks. All are large fowl and one of the older ones is a Brahma so she is huge. Our older chicks were terrified of the babies. They all ran to a corner to hide. One of the little olive eggers charged the big ones and even jumped on their backs! It took the older ones about 20 min. before they were back to roaming all over the brooder. We watched them carefully for the couple of days. One of the older ones, pretty sure it's a he, did peck on the babies every once in a while but the little ones didn't put up with it and pecked back. It might depend on the personalities of your older ones and your little ones. It worked for us and we now have 9 little chicks ranging from 1 1/2 weeks to 4 1/2 weeks living together happily in a converted baby crib brooder!