This reminds me of the time when I added new chickens to my flock.
I had two human- friendly RIR chickens( hen and rooster) in my coop outside.
The rooster adored me so much that he would jump in my lap.
Anyway, I decided to add 10 more RIR chickens to the flock.
I raised these ten as babies, then when they were old enough, I took them outside.
First, I knew the older two chickens would bully them since they were new and were pullets and cockerels.
So what I did was switch their living quarters.
The babies took the coop, and the older ones took the babies' old brooder, which I took outside and placed it near the coop lined with cement blocks at the entrance so it can block the big ones from escaping but also allowed ventilation from the gaps in the blocks.
Everyday I would let the big ones out to forage around the yard, while I went inside the coop to feed the babies.
Funniest thing was the older ones were surprised that new visitors had now occupied their coop.
They were so jealous that they spend most of the day trying to get inside the coop in order to attack the newbies, but no avail.
They can see each other through the fences of the run, but cannot touch each other.
Of course, one day I had to put them together cause that day was a very hot day, and the big ones couldn't stay in the brooder with the hot sun cooking them inside.
When I did put them together they did in fact pecked them, but not to the point where they drew blood.
I assumed it was a pecking order thing.
Now they are getting along fine.
Occasional pecks here and there, but that was because the older ones are protective of their food.
The babies know to give them space when they show up.
Maybe for you to get your chickens to get along is to get a crate, like a dog carrier and have them see each other but not touch each other for a few days.
Have you tried something like this?