I always introduce chicks when they are fully feathered, at least. I will have them in a large dog crate inside the large run. I keep them locked iin for a day, then open the door and let them go out and mingle as they feel comfortable. They usually roost in the dog crate for at least a week and I shut them in there at night. They will gradually start to put themselves to sleep in the big coop when they are ready, and when that happens, I just remove the dog crate. Iive never had any problems introducing new chicks to the flock.....but my run and coop are more than big enough, so there's not much need to figt due to overcrowding. Ii think that's where a lot of issues stem from.