I think it depends on the breed, and the breed mix. For example, I was told by a top orp breeder that you can keep bantam orps and large fowl together easily. I have done this and it worked out perfectly fine. I then went on to keep LF orps with bantam polands (good mix although LF males need some LF distractions!) and also with small LF (effectively large bantam) Sussex which worked fine, although I can't say it was the most cohesive flock ever, they tended to keep to their own breed. However, I have also looked after LF hybrids mixed with Pekins and bantam orps and the small birds kept their distance rather, although it wasn't constant bullying, mostly just around the feed pot. Overall, I am happier keeping sizes separate, but space sometimes dictates otherwise. My LF orps were all docile, I would worry more if I had a feisty breed. Perhaps I should also say my experience only extends to birds kept on the range for much of the time.