I have a mixed flock with roosters and hens of both sizes and have never had a problem ... well, I guess that depends on what you call a problem! I worried about the big roos trying to breed my banty hens, but that was never an issue. I have plenty of big hens and the big roos find it more comfortable to mount them. However, I set and hatched about 30 eggs last month - 10 banty eggs and 20 or so eggs from my BR hens - and of the 16 "large" chicks that hatched, all but 4 are undersized and have feathered legs! Well, my big roos are RIR (smooth legs). My most amorous little roo is a red cochin banty named "Lil Red" (keep in mind here that feathered leg genes are incompletely dominant). Well, these chicks have sexlinked color characteristics (males with white dots on the head, females without), so obviously dad was red in color, but as they grow it's becoming increasingly evident that they are not standards! Lil Red has been a busy boy! And yes, he will face down any of my big boys and walk away the winner. He's just plain meaner and more determined than they are!