Here's how I merge baby chicks with large chickens.
I begin when the chicks are just two or three weeks old. On nice days, 75F, the babies get taken out of their brooder and placed in a chick pen inside the main run. The chicks have their crate I transported them in inside this pen in case they get cold and need to snuggle for warmth, or to get some nap time. The big chickens get to know the chicks and vice versa. The chicks go back into the brooder for the night.
Around age four weeks, I open small pop holes that I've built into the sides of the chick pen. The chicks can fit through, but not the big chickens. The chicks begin to explore the main run, and get pecked and chased, and they learn FAST to run back inside their pen for safety.
At six weeks, I move the chicks from their brooder into the coop with the big chickens. This is the trickiest part. After the hens have finished laying for the day, I lock the big chickens out of the coop and move the babies in. They run around and explore the coop, and make themselves at home. At dark time, I place the babies on the perch and I let the big chickens in. I stick around to referee in case there are any problems, but usually they roost without messing with the chicks, which I've placed down at the far end of the perch.
In the morning, the big chickens chase the babies out of the coop, and they know to run right away into their chick pen for safety. They spend the day like they've been doing for the past several weeks. But at bedtime, I need to round up the chicks and show them how to go into the coop.
All chicks are different, and so are the adult chickens. You just need to play it by ear as far as addressing any problems as they arise. It varies as to how long it takes for the chicks to learn to go into the coop at night on their own, but basically that's all there is to it. You need to supervise roosting time for a couple weeks until everyone is comfortable with each other, but usually the adjustment isn't too hard.
This has worked very well now for four batches of chicks over the past five years.