I find that if they start out together they seem to do just fine. The little ones learn quickly that they need to get out of the way when a big girl comes by. The one concern is if you have a standard roo with bantie hens. You need to be careful as the roo can break that little girls legs etc if he tries to mate with her. I don't keep my bantie hens with the standard breeds for that reason, but if I didn't have a roo and they all got on I would probably have them all together.
I have my banties running with standards....Makes for interesting guessing as to what you're gonna hatch out later...but if you want them to breed true you separate the ones you want to breed true from the others.
The only picking problems I have had are the "high strung" personalities vs the ones in "chill mode". The chill ones (no matter the side) tend to get picked on.
The picking only tends to happen in winter when theyre cooped up more, too.
My banty cochin roo (Fluffy) is my top roo, and he weighs less than a third of my other roos! He is the king of the coop, and he is the sweetest brooder, too!