The horses that are usually the most aggressive when new horses are introduced are not the leaders in the herd most times. It is the lowest one or two that will fight the hardest as they don't want any more bosses.
Just throwing that out there.
Interestingly enough, years ago when worked at TB farm, we had an extremely big mare, who was very old at that time. She was always getting run off of her bucket by the others, who would hop around from bucket to bucket, like a litter of piglets.
I had at the time an extremely obnoxious small QH mare, a real witch but low on pecking order herself. For some reason she was put in with this mare grouping.
The big TB mare, Upturning, and the QH mare teamed up and basically took a stand. The QH mare was a kicker, but was too small to intimidate the other TB mares, and the TB mare who was 18.2 was too old and frail to battle the other mares. But together?
Katie bar the door.
Upturning would walk up with the QH mare and can NOT think of her name right now, and the two of them claimed 3rd bucket out of 15 or so....and they ate fine together the whole time they were there. Not one horse challenged them either.
Pretty interesting to see how they figured out a way to get to keep their bucket. We just put in double the feed, and they each shared it equally, no fussing, and no hogging of the bucket at all.
Other mares gave them a wide berth too. QH mare was maybe 15 hands or less.
Horses make me a better person.