Well, if you go by the rule of thumb of starting them at 95 degrees under the light, and raising it up every week so the temp goes down 5 degrees every week, after 7 weeks or so, they should be at about 60 65 degrees anyway, so most house temps are at least that anyway so the light Is rather superfluous. Removing the light then can help get them to feather out the rest of the way, which is when they can go outside... that part depends on their feathers and how cold it gets at night in your area.
Mixed in with an adult flock is a whole nuther matter. That takes some effort, the old birds will very likely try to aggressively insure the n00bies KNOW beyond the shadow of ANY doubt they are below the bottom of the pecking order
Id house them side by side for a while, but protected in their own section, then slip them together after at least a week or so under the cover of darkness. It can get nasty though, so be prepared to let them work it out and treat injuries.