I do the samething as gssw5. My last addition is a 22 yr old gelding(Doc). I kept him in a pasture separated by a fence for two weeks then brought in one horse at a time for a couple days each. They each tried to bully Doc as they entered and each got double barreled each time they tried to push him. He did not go after them as he just wants to do his own thing without being bothered. Finally the lead mare came in and also tried it with the same result. They have been in the pasture together as a herd now for 6 weeks and they just now are getting to the point where the upper 3 do not push him except at feeding time while waiting for their turn to go in their stall. He does a quick spin to give them a view of his rump being loaded to deliver a kick which usually stops the aggression. Occasionally the lead mare with take a kick then try to deliver her own which sends them both running from each other throwing half bucks.
If you can separate her then do so as was stated earlier. It just makes things easier on her and the heard. It will eventually work out as is, but takes longer.