Barred Plymouth Rocks are usually a bit bossy in the flock, but that's just them. They are rarely at the bottom of the pecking order. I love that breed and the barred variety in particular.
You have a huge age difference. That will never change and they will have to eventually establish a relationship, but the younger ones will never be over the Barred Rocks because they came later. They'll have to work it out and sometimes it looks rough.
As long as blood isn't drawn on the body of anyone, let them deal with each other. And be sure the younger ones have plenty of room to get away from the older ones and can get to the feed and water.