How old is this cockerel? Is he 18+ weeks and sexually mature? If he's not mature yet, treat him the same way you'd treat a hen you were introducing and expect him to be bullied by both hens and the other rooster.
If he's mature, since he's already been there for a week, I'd just put him in with the others. Make sure they're outside in the run so there's plenty of room for the loser to run away, and let them get on with the business of figuring out the pecking order. There may be a rooster fight. As long as there is plenty of room for the loser to run away, there should be no blood and no problem.
When I get new roosters, they are quarantined WAY out of sight and sound (and wind travel of possible feather borne diseases) for a month, then I just drop them over the fence during the day. The alpha rooster comes over to assert his dominance, and they decide whether or not it's worth fighting him. Usually, not. They work out their differences over the course of a couple of hours, sometimes some feathers are lost but blood is not, and all is calm again. Rooster pecking orders separate from and are a lot simpler than hen pecking orders, which tend to be somewhat subtle.
A couple of things--your bantam might win the fight, and your Orp might not be alpha roo. As long as they are OK with this outcome, it shouldn't matter at all to the flock. Also, you can't make specific hens belong to specific roosters. When you keep more than one rooster, they get to split up the hens, not you.
If for some reason one of the roosters isn't reasonable--they won't let the loser rooster run away, and box him into a corner to draw blood, or won't let the fight go even though the loser rooster is clearly showing submission by staying out of the way--then I'd get rid of that aggressive rooster (the winner).