I have two year old White Leghorn Banty hens and six, one month old (various breeds) chicks. The chicks spent their first weeks in the garage, first in a brooder, & then at two weeks, in a 4X8X3 chicken wire enclosed pen. The Banty's have access to the garage, and came in to have their afternoon preening alongside the chick's run. So, everyone had an opportunity to see one another.
At four weeks, I moved the chicks out to the coop. I did this after dark, something I would definitely recommend when integrating a flock. It makes sense - after night fall, when chickens have gone to roost, they are quiet. Also, the only light source involved was a flashlight. The chicks complained until they were all united. The Banty's watched quietly from their roost.
Ironically, I am usually awake before the chickens; the following morning, I overslept. When I went out, it was business as usual. The Banty's popped out of the coop, as they do every morning, for their corn. The chicks started milling around. They went to the feeder & waterer, and began exploring the coop. I opened the door to the run, and they went right out & worked their way down the ladder.
The Banty's free range, and I usually leave the coop door open during the day so they can go in for a "mug up" if they wish. That morning, I left the door closed, so the chicks wouldn't be loose. The chicks had a waterer outside; a feeder; a roofed, yet open structure to get under for shade. I put a flake of mulch hay up near the coop so they would have a place to "nestle in" if they wanted, which they loved, in turn with loving other areas of the run! I was around a fair bit that first day, and let the Banty's in to the coop/ run several times. The Banty's mixed with the chicks out in the run several times - without major drama, and certainly no pecking. The Banty's did (& still do) chase the chicks off if they encroached on their personal space too much.
The first two nights I had to bring the chicks in the coop - they did not come up the ladder on their own. I waited until the Banty's were on the roost before bringing the chicks in. By the third day/ night, the chicks were coming & going from the coop to the run on their own. I leave a waterer outside in the run, but if the chicks want to eat crumbles, they have to come inside the coop, which they do. During the day, if the Banty's are not in the coop/ run, the chicks come in the coop and get up on the big girl roost bar to preen & nap. You can almost see the look of satisfaction on their little faces. By the third night, at dusk when I went out, the Banty's were on the tall roost in the coop and the chicks were in the coop nestled in the shavings under the Banty's. Somehow, the chicks are managing to avoid getting pooped on, at least I haven't seen any residual poop on a chick! I've been placing the chicks on a second roost bar each night, but they fly down.
The only downside to the whole thing has been that during the past month, first one, & now the other, Banty has become broody. Plus which, I've still wanted to give the Banty's access to their nest boxes in the coop - I don't want them to start laying in unknown places, although I'm pretty onto all of their hiding holes. So, I've had to be, but by choice, pretty involved in policing the chicken's lives. They don't seem to mind. The chicks are quite friendly and the Banty's are still laying...though the one has a bad case of broody still. I put boxes in the nest boxes at night and for part of the day; part of the day, I let the Banty's in to be with the chicks and to lay.
I'm sure some would condemn some elements of my flock integration: not enough quarantine time; chicks eating layer mash at four weeks. None of the birds showed/ show any sign of illness. If this were 100 years ago, what would the chicks be eating at four weeks of age? Cracked corn; worms; bugs; gravel; etc. - exactly what mine are now eating. And layer mash.
