I had 4 BO pullets, 1BO cockerel and 1 silkie cockerel all raised together living in harmony just fine. At about 6 mos we had to remove the BO cockerel because he was too aggressive while mating and nearly scalped one of the pullets. We then had the silkie cockerel with the girls with no issues till about 8 months when we had to rehome him due to his aggression towards my kids. So while we got rid of both roos, I don't think size differences were ever an issue. And actually one of the BO pullets was besties with the silkie cockerel and would hang out with mostly him and not the others so much.