I have a wire dog kennel that I put chicks in to introduce them to the flock. I put it against one wall in the hen house so the chicks have a place where no one can peck them through the wire, and after the big girls start ignoring the little ones, I open the door a crack so only the little ones can go in the cage and they still have a refuge. I let the little ones out while I'm sitting out there in the coop, and babysit to make sure no one is picked on terribly. After a few tries at that, everyone seems to mix just fine.
This time, though, I've had my chicks in a separate pen outside. The big girls free range, and the birds have been able to see each other through the poultry wire on the door to get acquainted for the last few weeks. When I opened the door and let them out for the first time this week (they were 7 weeks old) there was no picking, and everyone seems to be mingling beautifully. But with my flock, there are ALWAYS new little ones being introduced, so maybe the girls are more used to it.