Once chicks are fully feathered they go to the coop. For the first week in the coop with the big chickens I put up a partition so everyone can see each other but not get to one another, after a week or so I turn everyone loose together. I find that adding a single bird is way harder than adding a small group. A small group will stick together and assimilate into the main flock faster, where I find singly introduced birds have a longer period of upset and distrust with and from the flock. I find in my coop that the chicks stick to themselves and avoid the larger chickens and the adult chickens do the same.
In September I added 5 fully feathered chicks and their mom. The chicks all stuck together as their own tiny chicken flock until just recently. The chicks mom fit right in with the original flock as soon as she stopped brooding the chicks (@12 weeks) A month ago you could look in they yard, or coop and see a flock of adult chickens in one spot, and a flock of adolescents in another spot, but once the pullets have started to lay they are all one happy flock...there wasn't any pecking, I have a rooster that won't let anyone fight, but everyone knows their place with just a look from a higher up chicken most of the time. Jeez that was a super long answer for no reason...anyway my chicks move to the adult flock as soon as they no longer need heat.