As chicks and juveniles most pullet breeds mix ok. It's when they reach adulthood anywhere from 18 mos to 2 years that the battles begin. Sometimes a very hard moult or a broody period will bring on aggressive behavior and once started sometimes never stops unless the hen is isolated or re-homed. My friend has a "hen-itentiary" pen where she throws her combative girls until they either calm down in a few weeks or are sent to freezer camp. I'm not talking about normal flock politics where there is a matriarch leader that keeps others in line - I'm talking about damaging behavior like tearing out feathers, digging claws into others, or chewing off the combs or wattles of other flockmates, etc. We had an alpha White Leg leader that was wonderful for 3 years and then suddenly went ballistic on the flock. She had to be re-homed and today is still assertive but in a flock of equal peers.
OK. What is a BCM?
Thank you for posting that about Mediterranean breeds. I get so frustrated when people ask me advice on what to get, then decide they can keep Production Reds, commercial white Leghorns, and a Silkie, Buff Orpington, or Polish together. This year several people on a local group have reported seriously injured birds, and one Silkie was scalped, most of the skin on her head and back removed, and she died, and it took a while because her owner didn't understand that a chicken missing that much skin is not going to live and didn't promptly put her down. All goes well for a year or two, and then disasters happen.
Aggressive birds may start by eating all of the feathers off of a bird. They may graduate to eating combs, wattles, tearing out a bird's genitalia and eating it while she is laying, stripping skin off, and eating the flesh underneath. You really don't want to go there. Sometimes you can save the situation by taking a really sharp dog nail clipper or toe nail clipper and trimming off about 1/4" of the upper beak. Although this will grow back, some hens give up after a few days of "firing blanks" at the other girls. Omlet sells bumpa bits, which can also work. We had a problem adding younger girls to the flock that would ebb and flow so I tried pinless peepers before going back to that Ohio invention of the 1930s - beak trimming. I don't like pinless peepers because not only do they interfere with vision, but I watched our biddies help each other remove them. Within minutes I had Black Star hens reaching out to each other and trying to pull them off of each other with their feet. Within a few days they had successfully removed them.