When I hear a chicken sound and not a chick sound, that's when I consider introducing the youngsters. A chick sound (any peep) means still young and submissive and easy pickings.
I used a wire dog kennel to introduce two new birds to the flock. Set it up in the coop, provided food and water therein. The birds could see and be near each other, but no physical interaction. And after a long week (about 8-9 days) the kennel door was opened, birds wandered out (curious...