I finally have all the chickens together and after one week all is well! They are now (2) 10week olds and (4) 21 week old. I split the run and when the little ones were 5 weeks old, they went into the split run using a large dog crate for a coop (with roosts). At 9 weeks old, we built a "shoot" with a door on the side of the spit run, and cut a loop into the wire splitting the run. That way we could open the door and let them mingle, but the young chicks could run back to safety through their small hole. We only opened the door when we could supervise the interaction. Tried this at 8 weeks old and it was too soon. At 9 weeks we tried again, it was rough and the young chicks ran back to safety a lot. The worst were the lowest on the pecking order among the older hens, they felt more threatened. After about 3 days of mingling, we took out all the wire and the crate and they were all free. We have enough safety zones with a low branch, roost, and an underneath the coop space, that the fast running youngsters were able to avoid too many peck. None of the hens had the inclination to chase very much. Mostly there was opportunistic pecks. The first day free, I locked the younsters in the coop by themselves for a couple hours in the morning so they could get use to the coop. Where to sleep? We were going to move the dog crate back in the first night, but I decided to do the "sneak them in the coop at night routine". I waited till the older hens were asleep, and by then the babies were kinda panicky about not having someplace to go. So, I snuck them in under the dropping board. After about 15 minute, I opened up the coop and popped them onto the roost.... It worked! I went out at 5:30 AM the next morning and opened the pop door in the dark while all the chickens were asleep. When the sun came up, the older hens came out and the younger hens stayed inside for another hour, eating in private inside. Then they came out, got chases a little, then things settled down. I put extra water and feed in the areas the younger chicks could trapped into. The next night, they walked in by themselves after the older hens had settle down! I am still going out in the early early morning to open the door so they are not trapped inside my small coop with the older hens. They tolerate each other more each day and nobody has had any injuries, just a few feathers lost. Plus I got my first egg this week! It was a struggle, and I wanted to share what worked for me and at what ages. All the actions taken were from ideas read right here on BYC! Thank you all so much for sharing your strategies and helping me through this. It was harder than I thought getting new chick into an enclosed space. Hope this helps someone new who is needing to do this. Cheers Everyone!