I subdivided my run, 1/3 for the youngsters, 2/3 for the adults. Did coop about 40-60. I remove door inside coop and gate outside in the run divider when they reach same size. Have never had first problem this way. I only install gate and inner door every 2 or 3 yrs when I order new pullets and they are ready to move from our foyer in their large box to the coop/run.
One of my tests (although now mostly I raise chicks with broody hens):
When the chicks will run right up to the fence (on their side to start) and stay there even when my greedy girls are getting their scratch treats for the day -- that's usually a great sign that they're ready to be together.
Critters (chicks, mice, whatever) that scurry away and screech and make all sorts of fun noises become a fun sport to chase in the chicken world. Half the issue is the chicks becoming confident. If they run away the second they're challenged, the hens will chase them. When they act like they belong, usually things settle down.
Things are at their most charged when there's a valued treat to compete for. Thus- when the littles are willing to stand their ground and aren't startled by the hens anymore, generally it's a good time to combine them. I don't mean when they're really little and don't know any better- I mean after they've been in with the bigs for controlled stretches of time.
I believe the hens are just letting the chicks know that they rule the hen house, it's all part of establishing the pecking order.
It could get severe, but so far it has been going good for my chicks. Nothing to severe, I did have to interfere once