Just wanted to share my experience with integrating ... in progress right now. I have a mixed flock of 2-yr-old RIR and BPR, 35 hens and 2 roosters. These were all raised together. This spring, I received 29 black australorp and 8 old English game bantam chicks, also brooded together, but in a separate building from the adults. At 6-1/2 weeks old, I moved the Aussies and Bantams into a run and coop of their own, sharing a fence with the big birds - so they could all see and meet each other without touching. For a week, the Aussies and Bantams stayed in their own run, while the big birds had their run and free range as well.
The fence I have separating them, and from the outside, is chain-link. Not the best choice for chicks, but it's what I have. I worried for the Bantams at first, as they are still small enough to race freely through the fencing like it's not even there - and they did, from day One. My worry was for nothing - as the Bantams are just fast as lightning and never had a problem coming and going freely as they pleased. After about a week (just a few days ago) I opened up some gaps along their shared fence by putting some bricks under the edge of the fencing. Big enough for the Aussie chicks and growing Bantams to pass freely through, but the big birds can't follow. It's been working very well!
Today, most of the Aussie chicks and Bantams were mixing with the big flock. NONE of my 37 adult birds has bothered them one bit. They scratch and range (in the run) side by side. A couple of the big birds squeezed themselves through the holes in the fence into the chicks' run - but again, no problems at all. Tomorrow, if the weather holds fair, I'll let a group of the big hens into the chicks' run and see how that goes. If all is well, Saturday I plan to open the gate between the runs and let them all mingle in both runs.
In about a week or two, we're donating two dozen of the adult birds to a couple different families - so this should help (I hope) with establishing a new pecking order inclusive of the Aussies and Bantams in the remaining flock.
And ALSO I have four 6-week-old rescue chicks that we received two weeks ago - They're currently in a THIRD run and coop we have, with 26 jumbo Cornish X chicks (same age as the Aussies and Bantams), who have a date with their destiny next week. Except the orphans - I'll move them to the newly-vacated smaller coop and run, and do this integration thing all over again for just those four.
Thanks to those who suggested all the tips for integrating chicks with adult birds --- it's working!!!