I would by no way suggest I am an authority, but this is what I have done and it has worked good.
We bought 5, 2 day old hens from the local feed store. Then 4 weeks later hatched 6 more. When the original hens were 6 weeks old we put them in the chicken yard, we also free range, and they spent the night in the big coop. When the second set was 4 weeks old we put them in with the older girls. Watching carefully and putting them in their own cage at night. Then after about a week we put them all in the big coop at night. This has gone on for 5 cycles of chicks. For a total of 30 over 5 months.
We now have 13 that are 4 weeks old that spend their day in the chicken yard, but their nights are in their own kennel, not in the main coop. In about 2 weeks I will move their kennel to the coop for a few nights and then turn them loose to sleep in the big coop at night.
I love to spend time with my chickens and so they think I am the head rooster. I do not allow any fighting or picking. I chase my roosters with a broom if they get too rough and only allow fighting to go so far, establishing pecking order. I am sure when I am not there some fighting goes on, but I have not had any problems. And I have 8 roosters!
I only buy my chicks and hatching eggs from reputable people. I keep them quarantined in the garage for two or three days when they first hatch or I get them. If any were to die it would be longer. I feed medicated feed the first week and then corn, free range, scrap and some commercial feed. Our chicken yard is 100 by 100 by 100 attached to an open shed, that is 20 by 20, were the duck houses are. We also keep our ducks in with the chickens during the day, but in their own houses at night. Our coop is a 12 by 12 mini barn that we build ourselves just as a coop.
I haven't introduced older birds to my flock. I prefer to only do chicks. But I would follow the same method.
I hope this helps, Tracy in Indiana