You should never put young new chicks in with full grown birds, that is the problem in a nutshell, I'm sorry to say. The full grown ones will try to kill them, and often do. Let the babies grow up separate. You can't add to a flock without taking great care at integration. It upsets the true pecking order.
This must not apply when you ad grown hens to chicks? I bought 4 hens yesterday. Two are RIR and over a year. They pretty much ignore the chicks. The other two are Barred Rock mixes and are 5 months. They have chased the chicks some but not seriously. The chicks are around a month.
I do keep a close eye on them and the chicks still come in at night, but so far they are getting along well. Hopefully it will stay that way.
Birds are territorial, so adding adults to young ones may be less problems until the old ones see that they can take over, take over, and want to put the young ones in place. However, if you have enough space and lots of room, it might not be so bad. I still wouldn't make them share the same sleeping quarters. I do let different age groups free range though, but they stay in each others general coop areas.