Had 24 brood-mates , 4 different breeds.  Never saw it.  They were all raised in same box, but I had to add 2 more boxes as they grew since my coop was not done yet.  I did notice a RIR being bullied by two golden comets in one of the boxes, so I replaced the RIR with another GC and that fixed it.  After they got into their coop, I kept them confined for another week while I finished my pen.  After that, I noticed a runt Barred Rock who was limping.  It never has gotten better, but I see no consistent bullying.  No blood, so I thought it was a mid air collision or something.  The runt does stay by herself a lot, but a part of that is not wanting to be knocked down while scrambling for treats.  She has learned to go into the coop a bit early to get her spot on the roost.  (I always carry her special treats and pick her up to feed her in front of the others)  Twice I knocked a golden comet off of the roost and flipped off the light for bullying her as they settled in for the night.  Seemed to stop it.  I see them mixed everywhere a lot it seems.  They sometimes cluster a bit, but not a solid and constant thing.