I've introduced new chicks into my flock without incident after they reach 10 weeks of age. I do have a portion of my henhouse segregated so that I can move the chicks from the brooder into the house when they are 4-6 weeks of age. They stay in this fenced off section until they are at least 10 weeks, then I let them loose into the flock. I've done this twice without any of the other hens or roosters bothering the new ones. I've discovered that the new ones will stay separate from the flock until they mature to around 4 months of age, then will fully integrate into the flock.