Have you heard of the panic room method of integration? It makes merging young chicks as young as two or three weeks with older, larger chickens a breeze, safe and easy.
The way you do it is to provide a safe pen in your run for the babies. Back before I brooded outdoors in my run, I would start taking the chicks out of their brooder in my house to let them romp in a pen in the run during the day. Then I would take them back inside when things cooled down.
After a week or two, I opened portals from their safe pen so they could start exploring the world of the adults. Now, the chicks are brooded right in the run and the portals are opened when the chicks are between two and three weeks old, and is it ever fun to watch them screaming at high speed around the run just a few steps out of reach of pursuing adult chickens, and they zip back inside their safe pen to safety and the adults can't touch them
My article on outdoor brooding features photos of my panic room setup with the portals. It's linked below my post.
