Three times now, I've had the experience of introducing new kids to the established crew.
The first time it was an adult hen I was introducing to just one existing hen. I was clueless, and I simply popped her into the pen. The two girls stood motionless, beak to beak, until the home hen hauled off and beaked the new girl right between the eyes. They were best buds from then on.
A few months later, I obtained three four week old Brahma pullets, but had the good sense to ask for advice. I was told to keep them separate from the older hens for at least three weeks, but let them get used to one another through the safety of a poultry netting barrier. (No one told me about quarantines, but that's a good idea.)
The three Brahmas were still being heat weaned, so I would take them outside to their pen enclosure every day and bring them back indoors to their box with the heat lamp at night. When it came time to merge them into the coop, I put the three youngsters in first about two or three hours before it was time for the two adults to roost. At first I tried it the other way around, letting the big girls roost first, and then putting the babies in. But the babies were terrified of the big girls and set up such a screeching like they were getting slaughtered, I took them back out, and tried again the next night.
I did this for just a few nights, locking the big girls out for the three hours before dusk, letting the youngsters get used to the coop and roosting on their own. The three Brahmas even opted for the premium spot on the perch, and the two big girls accepted it. Soon everyone was going in to roost all by themselves with no problem.
However during the day, I created a small "panic room" in the pen for the youngsters with a couple of small entrances too small for the big girls to fit through. I put their chick feed and water inside so the big girls couldn't eat it all up. They very quickly discovered they could escape bullying by running into the panic room, and boy were they fast! When the youngsters got to be almost the same size as the older ones and appeared to be handling the pecking order, I got rid of the panic room.
This past spring I raised a new batch of Wyandottes from kiwi size. While the babies were residing in a cardboard box in the house in their first few weeks, I added onto the coop. It was originally 4' x 4', and I added on a new 4' x 8' section. I installed a temporary barricade to separate the old section from the new, each with its own roosting perch. I sectioned my pen into two parts with a tiny portal between the two sections. After the babies were heat weaned, they would spend the day in the pen, and I'd carry them all inside to their section in the coop for the night.
This went on for about a month, and I took down the barricade in the coop and opened the tiny portal in the pen between the sections, but it did not go well. This bunch of newbies, even though there were ten of them and four of the big girls, just couldn't seem to hold their own. So I put the barricades back for another couple weeks before trying again.
Every bunch is different. The age doesn't seem to matter as much as maturity and size and whether you have a bully that insists on complicating matters. You just have to play it by ear. But having the safety of separate quarters while they all get used to one another is the most important thing.
