The worst thing you did was intervene, they probably owuld have sorted it out then.
Have you tried introducing them 1 at a time? My policy used to be that new horses would spend a night in the stable next to the boss (used to be stan) so they could talk over the wall and then would be turned out with stan for a few hours. Then when they were settled with stan I turned everything else out too.
Normaly when the boss has a accepted then everything else will