I get my chicks in late spring so it's warming up and drying out by the time I'm ready to move them to the coop, depending on the temperature outside I put them in the same shed as the older ones at 6-8 weeks, when I no longer need to provide heat, in my large fowl shed I have a coop within the shed that I put the chicks in and keep them penned for about a week until they are comfortable, in my bantam coop they go into a wire circle. I start letting them out of confinement after they look more comfortable in their new pen, starting for short times than lengthening how long they are out until I'm just locking them up at night, mine all free range so they travel further every day. The adult chickens mostly ignore the chicks because it's a normal thing for chicks to show up in a flock, they occasionally peck them but I've never had anyone go after them, that first week of lock up helps for the adults to get used to seeing them.Then one day I stop locking them up and they are on their own, usually by 4-5 months.
I have around 15-20 bantam who have their own coop and around 30-40 large breed, they all free range, my large fowl are in a 40x40 pole shed, the bantam have about 8x10. I mostly introduce groups of 8-15 in the large fowl, and 3-6 in my bantam every couple of years.