Admittedly, I've only introduced new adult birds once, but here is how I did it.
I started out by letting the current flock free range (as usual) with the new birds in a fenced-off run in sight of the current flock (attached to the old coop).
When evening came, the current birds headed back towards their old coop, which was attached to the run.
There was a little bit of pecking between the alpha bird of the current flock and the alpha bird of the group that was being introduced (lasted maybe a minute, no injuries).
Since I had just built a new, bigger coop for everyone, I put all of the birds in there when it got dark. One of the newbies was cuddled up with the old girls within minutes!
I plan to do something similar when my chicks are big enough to join the adults. They are living in the old mini-coop with attached run, so when everyone is outside they will all get to see each other and get used to each other. Once the time comes, it's easier to put everyone together that way - they are already used to each other.
As for breeds, I don't know how much it matters, but what I have is in my signature. The 3 Marans were the original birds; the other 9 were new.
Edited for spelling - I'm tired.