Hi there!
Well, I've integrated into my flock a couple of times. First time when they were just a couple of weeks old with absolutely no problem
The second time, the older ladies were about four months old, and I integrated a couple that were two months younger, and it was terrible!!! I had kept them where they could see each other, but not interact for the first two months, and when I took the fence seperating them down, they woke up the next morning and you would have thought that world war three had started in the coop.
I am currently in the process of a third and final integration. I have them seperated, but they free range together. Every time one of the older hens come toward my chicks, I get between them, and let them know that these are my chicks. They are now about 8 weeks old, and i have been lessening my interaction in the flock. I am only having problems with the two broody hens chasing the young ones now, but they chase everyone. I'd say this integration is going as smooth as the first one did, but they are still seperated at night and when they are not free-ranging. Since this integration is not complete, i can't say it works yet.
Good luck to you!!!