When one broods out 12 chicks from the same hatch, they all "know" each other from day one. The sort out their pecking order rather matter-of-factly and life goes on. When you mix birds not brooded together, the birds' instincts say, "this is an outsider, an intruder". Being different ages and sizes places the little ones at a distinct disadvantage as they cannot physically match up to the chest bumping, stare downs, and harsh pecking that occurs when birds have to "sort things out" in the pecking order.
If they can spend time together, separated by a net, I find this helps, somewhat. I also like to have them out in the yards together, from time to time. They'll avoid each other, or if the "inferior" approaches, it does so at some risk of being driven off. Slowly, carefully, the flocks CAN be merged. Patience is the key. Familiarity parties often work best if held in late evening too, as everyone is little tired and want to go to roost for the night.
The day finally comes when everyone gets along. Sort of.
You just never know. Sometimes it is easy and sometimes it is more difficult, due to personalities and type of birds. It never quite goes the same way every time.
