Hi, there
If you put new hens in with an established flock, you might get fireworks but it's nothing to do with mixing the breeds. There are two schools of thought on this - stick them in and let them get on with establishing a new pecking order or a more softly, softly approach, which has always worked for us.
We pen the new hens in sight of the flock and give them corn together, but with the new ones safely penned. After a week of so, we put the new ones in with the old ones at night - the idea is that by morning, they don't know the difference. It's always worked for us although ours freerange, so any hen getting a hard time can just leg it.