I'm trying this myself. Last year I tried to introduce 3 adult, but young, birds to my established flock of 6 birds. They almost pecked one to death.
Now, the older flock is down to three. ( I lost 3 this year to old age, I think.) So the numbers are even: three to three.
One if the older ones is really making sure that they all know that she'e top hen. They are all so spoiled and sweet, I hate to see the fighting and fussing in the coop, but I need to do this if I want to add more chickens to keep some that lay well.
They have free ranged almost every evening, amd I kept the coops side-by-side so that they would see each other safely.