After quarantine, and that means away from your flock, the best way I've found to integrate new chickens into the flock is to set them up into a separation/integration "meet & greet" area within your run. I have grow-out coop(ish) buildings, like the duck house which is only 3 ft by 3ft and 2 ft tall. (The ducks never used it...) It's raised up on concrete blocks. Then I take chicken wire or other poultry fencing and garden stakes, and put up a fence around the duck house but within the entire run. That's where I put the new chickens for about two weeks. They get their own feeder & waterer.
The newest and original chickens can all see each other, hear & smell each other, and even chest bump through the temporary fencing without any injury occurring. By the time I take the temporary fencing down, there is very little chaos when they can all mingle together.
It's like, "Oh, it's just you, not a complete stranger," and while the pecking order must still be adjusted, there's far less actual, all out warfare and fighting.