I use the playpen method. I have a large kennel in my run that I keep the younger birds in for a few weeks.This way you can introduce them to the flock and they can see each other without the threat of the older birds injuring them. I would not combine flocks until the younger birds are about the same size as the other birds so they are big enough to defend themselves. There will be some aggression to reestablish the pecking order once they are introduced freely but if they are being viciously attacked and injured or blood has been drawn, keep them in the playpen another week or so. How big is your flock and do you have multiple roosters?