What I did when I had several age groups of chicks(6 week age span) had one large brooder split in half (4 and 3) and another dog crate(2) so they could all see each other. I would take the older 5 out to a temporary day pen(neutral territory) and see how they got on. There was pecking and 'fighting' but as long as it wasn't severe I let them work it out. If there is a very aggressive chick I would bring them back inside to their brooder for a time out. Was able to integrate the 2 older groups like this, the 3rd group was the youngest. Those I built a 'safe room' out of a dog crate and put all of them out in the chick section of my coop.
