I just introduced 5 new ones to a flock of 6 others, and after 4 days it's gone mostly well. We had one too many roosters, so he had to go. But since then, things have gone really well. I put them together after dark the first night. Things are much more calm that part of the day, and there was a little positioning for the roost, but it was the best move we made. After that, there was some pecking, mostly by the roo we got rid of...some obvious reshuffling of the pecking order, but after only 4 days we let them out to free range yesterday, and there wasn't a single issue between them, and they all dutifully returned about an hour before dark.
We have a real mix breed, with only the 3 leghorns sharing breed. The others are all different, so I doubt that had anything to do with smooth integration. All but one is under 1 year and that might have helped. I also made sure there was a 2nd water and feed station, so that if the bully of the group doesn't want them around, there are other options. I'm extremely happy how things went. I hope you have the same luck!