That sounds about right, confinement can trigger some aggressive behavior towards the lower end of the pecking order, especially when jostling for roost space at night. Bantams mature around the same time as standards, usually you can tell by weighing them if you know the breed standards. I would say around 20 weeks like your EE's Your best bet is to let them just sort it out. It will be pretty dramatic for a week or so, but pretty soon everyone will know their place and coexist. At 10 weeks the bantams are not going to "break" if stepped on as easily, the worst you'll see is some pecking that may draw blood, that's normal and is quick to heal. You may want to coop them separate at night and let them co-mingle outdoors during the daytime to be safe until they can hold their own. The attacking is normal for introducing new members to a flock. Give them time. They will get it figured out.
