The rule of thumb I have read on here is when they are fully feathered and close to the same size but I push it just a little since waiting till they are would take a very long time! Too long for me to keep them separated. I wait till they are big enough to at least try and defend themselves (3-4 months) and give them somewhere they can run for cover if needed.
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.
It took just under a month for my young girls and older girls to get used to one another. I housed the younger ones inside the run in a cage I built with a dog crate inside for the night. Then one day I just let everyone out together and it was touchy for about an hour but after that, all was fine. The younger girls just seemed to know they were at the bottom of the flock and accepted it. It was stressful but not as bad as I thought it would be. This year my hubby is building me a new coop with a side wing for my newbies. They will get to live together but be divided by wire at night and they will each have their own separate runs next to each other during the day. One night I will just remove the wire and hopefully all will go well