I have 7 grown up adults. 3 are huge Buff Orpingtons, and 3 are huge White Plymouth Rocks and Delawares. I had 16 five week old chicks to assimilate. The chicks are Cochin bantams and polish, so they are Teeny tiny. About 1/4 the size of the orpingtons or rocks. But I still wanted to assimilate them, cos even if the cochin bantams got to their mature size, they'd still be so much smaller than the adults, so why not assimilate them slowly, but safely earlier? That was my logic anyway.
What I did was to move the chick brooder from inside my house into the adult chicken enclosure. This way, the adults could see, hear, smell and feel the chicks and vice versa, but neither group could actually peck or touch each other. I moved the chick brooder in there for 2 reasons.
1) To allow the 2 groups to know of each others presence and start getting to know one another. The adults could still go and come as they wish, free ranging and what not, but the chicks stayed put in their brooder in the enclosure.
2) To acclimatize the chicks to outdoor sights, sounds, smells, temperature.
After another week went by, I made a gap in the chick brooder (while in the sheltered enclosure), thus allowing them to go in and out but stopping the adults from getting in since the gap was only big enough for chicks to get through. This "panic room" allowed the chicks to roam around the enclosure, yet escape from adults pecking if they need to. I was teaching the chicks that this enclosure was where food, water, and safety was, and they should always stay close.
By the end of that week, I move the entire chick brooder into the adult chicken coop, where the adults sleep at night. During the day, the adults free range outside of the enclosure, and night time, they go back home to sleep. By moving the chick panic room into the chicken coop, the chicks learn to sleep with the adults being in the same room. The adults are slowly getting used to having the chicks there all the time, but having the panic room for the chicks means the adults can't attack the chicks. After one week of the panic room in the chicken coop, I remove the brooder entirely, but still keeping the heat lamp in there. Now, the chicks learn to perch and sleep like the adults. They begin by sleeping close together on the floor first, but after a while, they all perch on the roosts. No problems.
This system has worked for me tremendously well. After 3 weeks of seeing but not attacking, the adults are more tolerant of the chicks. When they first get together, they still do a bit of intimidating, and pecking for social order, but once the hierarchy is established, all is well again. They now hang together, sleep together, eat together, free range together, sunbathe together, dustbathe together, and there is peace on earth in our corner of the world.
I think the trick is to do things step by step, slowly and to persevere. A little pecking and intimidation is normal, but with careful planning and thought, it can be done.
Good luck to you.
Here is my flock and pack
Background, you see my Airedale Rummy and Kimi, surrounded by the young juveniles. In the foreground, is one of my buff orps.
Night time, they all roost together in the chicken house. The larger adults, being more senior get to sleep on the top roost, the younger ones settle for lower ones.
There are a few peckings here and there, of the big pullets telling the young ones to stay off the big girl roost, and they seem to obey. Now, some of the younger ones have wiggled their way up to the top roost too.
Good luck to you.