It is not nice to the bantam hens to keep them with standard roos.
But, while chicks, mine had no problem being together. All of my standards are hens, and they are probably up high in the pecking order, but not rough with the others.
I know someone who keeps all together: standards, bantams, hens and roos. She looses the hens for some reason, and there fore has a regular increase in the percentage of roos.