I think I would probably wait until they are full-sized at about 16-18 weeks. I normally start my integrations at 12 weeks, but do not allow the birds to start mingling until they are between 14-16 weeks. I keep only standards, though. With the little guys I would wait a bit longer to make sure there is the least amount of size disparity as possible. I might also try a longer introduction period than I would typically use because of the size differences. The roos specifically need to be well used to the sight of the new birds before letting them mingle. If one of your roos takes exception with the bantams, it could be deadly for the bantams. For that reason I would take the integration period very slowly.
Good luck.