I introduce mine gradually by putting them out in the main coop but sectioned off with wire mesh for a few weeks. That way the older birds get used to them being there and can't peck on them. I usually put them out when they're 8 or so weeks old, but it all depends on their size and the time of year really. Right now I've got two Leghorn chicks at 8 weeks old but they're still tiny looking so I'm going to keep them in the brooder another couple of weeks till they fill out a bit more.
As to when you can do it safely? I'd say that depends on your birds really. When you do finally put them all in together without mesh to separate them, do it at night once everyone's roosted, that way they will hopefully just wake up in the morning and totally accept the new situation without causing trouble. But keep a very close eye on them for the first couple of days and watch out for signs of trouble.