The best way to introduce babies is to take them out for brief field trips starting around two weeks. Have a secure pen for them with small mesh around the bottom so tiny heads don't stick through and get clipped by older beaks. Choose the warm part of the day and they can handle a little while without heat. This also helps acclimate them to cooler temps, a process that should begin by the start of the second week, so they can be weaned off heat in a timely manner.
I brood in my run, so the babies are already familiar to the adults. They start mingling with the adults at age two weeks, scooting in and out of their panic room as they get braver. They're fast little buggers, too.
Brooding indoors, you will need to transport the chicks in and out, but that's how I used to do it and it worked great. Any method you come up with so the chicks can safely get acquainted with the adult flock and the adults with the chicks will work. They need a week or two in proximity for this to happen before you let them mingle. The chicks learn by watching the adults, learning the different temperaments, then they are better able to cope with the pecking order later when they merge.
The younger the better. Small chicks are less threatening to adults and will have an easier time than if you wait until they are larger. Did I mention how fast they are?