I use separate "grow out" housing. For the first 3, maybe 4 weeks, I put up a temporary bit of poultry fencing to keep the kids separate from the adults. They can all see and hear and smell each other, and chest bump through the fence and all that safe interaction, but nobody gets hurt. Then I take down the temporary fence. By that time, everybody is used to everybody else. Sometimes the Big Chickens will have to teach the youngsters they simply must hang out on the periphery of any Treat session - no dashing in ahead of higher ranking ladies! - but other than that, and an occasional squawk as a Big Chicken puts a little one in its place, no major fusses.
They've got to get closer in size to the original flock members before it's all that safe to let them intermingle.
Now, I've got some bantam breeds, and that was interesting to watch, because the banties will NEVER be 'closer' to the same size as the original flock members!