I made that (ROOKIE) mistake and added one black australorp to my 2 gold buff orpingtons 5 days ago. It took until today for the orps to somewhat except her......but not fully yet. The matriarch hen is not the overly aggressive one, its the minion orpington that is the problem. When all three are in the run area of the coop, the minion hen starts to push, peck and plain out bully the australorp. When the door is open to free range around the yard they tolerate each other with an occasional charge, push or peck. I spent a lot of time watching and observing the 3 over the past 5 days, with the occasional referee to stop something when I felt it was getting out of control. 99 percent of the time I spent out by the coop I just had to let it be.
Good luck. They dont have to like each other but in time they will tolerate each other.