I'm a firm believer that horses need to sort their own pecking order issues out. While there are certainly exceptions (such as SpeedRacer's Conny mentioned above) I find the more you try to get involved or dictate the herd order, the more you prolong problems in the pasture. Some horses are by nature status seekers and dominant, while others are followers and more submissive. My herd never had a problem until I added several more horses. Monty, my rescue hackney pony, who is a very dominant little guy, Stiffler, who is submissive to the extreme, but attached himself to Monty so was therefore"guilty by association" in the eyes of my dominant pasture horse, Claymore. It threw a big wrench in my pasture order for about 2-3 months, there were a lot of scuffles, and Stiffler and Monty were both sporting a good number of superficial bites and scrapes. It actually settled down with the addition of 2 more mares to the pasture, and Monty getting over his David/Goliath issues. My pasture is now relatively quiet again, but we still have occasional food squabbles and two mares (Rain and Freyja) who are still going back and forth over the lead mare position.
Since you don't have the facilities to separate, to my mind you have 2 options - wait it out and see if they can organize themselves, or send the aggressive horse on down the road. Good luck with whatever you choose to do!