This is what I do. It works for me, it may not work for you.
I incubate a dozen eggs every three weeks. WHat hatches, hatches. Say.. nine on average.
Those go into a plastic tub with a heat plate. Starting about 10 days of age, they start getting outside time in a tiny chicken tractor I made from PCV and bird netting, weather permitting.
Not later than three weeks, but usually closer to two weeks, they get transfered to my grow out pen and attached run. The pen is in the barn, a door in the side of it offers access to a fenced run. There they join the birds from the last incubation. The birds from the incubation prior to that (8-9 weeks) get kicked to the adult flock.
The adult flock has two houses, a large run, and free ranges all day. They can see, and be seen by, the birds in the grow out pen all day long while free ranging.
Basically, my flocks are *always* integrating. The hatchlings with the juveniles 2-3 weeks, the juveniles with the adults at 8-9 weeks. Every couple weeks, theres a double move - just hatched into grow out, oldest grow out into adult flock.
Keys to remember:
1) Abundance is a social lubricant. Being seen eating, ranging, digging dust baths under greenery, drinking, etc w/o intruding on bigger birds' sources reeduces friction.
2) Pecking order is real. Even in the best circumstances, some pecking will occur. Blood spots on the comb are not a reason to intervene. Group attacks on the hind quarters, extreme feather pulling, open neck wounds - take the injured and move it back to the last place. ALSO - chickens don't have a strong sense of object permanence - if intergrating in the run, be sure to add tunnels, boxes, etc to block line of sight. Dead ends are bad. Narrow runs where a single aggressive hen can command the space are bad.
3) even with perfect integration, birds will still fail to use the nesting boxes correctly when they begin laying, at least half the time. and its better to have more perch space than less - at different heights.