I have a couple thoughts on aggression and herd dynamics having had experience with different herds as a barn manager and owner/trainer. I work with a mare now who I USED to call the "Battle axe" for obvious reasons. I've known her for years and I learned from her and my other mare who used to be more submissive, that the chemistry is based on who they are with. So as I moved them to different barns and herds, they changed in dynamic. The "Battle axe" when mixed with a herd she approved of was actually more or a "Lead" type horse. When mixed with a herd she didn't approve of, back to "Battle axe". My other mare had lived in several locations throughout her life. Generally being the bottom to middle rank in the herd. She was moved to a barn not so long ago that had a large mixed sex herd (of about 14). For the 1st few weeks she took a BEATING. I was concerned, then all of the sudden she switched to taking top rank and became a "Battle axe" for the first time ever in her life. She was the one to be reckoned with. So I had the chance to watch the horses interact in different herds and see how they faired in different situations. I do step into my herds and shape overly aggressive behavior when I am around. And have seen that it helps most when you are the primary care giver of the horses ie. they live at your place or in my case I manage the facility and therefore set the "tone" of the energy at the farm. I have seen this in action at many farms. Whoever the primary caregiver is dictates the energy and flow around the barn for good or BAD. If the person is timid and lacking leadership skills, the horses run the show. And that is where problems occur often.
I also have owned and worked with stallions. I take special care to keep them socialized and when matched with the right energy level geldings or other young colts, they actually do very well and get to play and rough house as boys can and should be allowed to do.
Horses in the wild select their herds and mates. I have seen over and over again, my show clear preference for one horse over another. Especially, my mares. They live with one gelding and are basically companions, then they live with another gelding and are madly in love with him. A Clear preference. I think when horses are under socialized or live a life where they don't have any interaction with other horses, their social skills are stunted. I have seen this with horse that previously lived alone and I had to integrate to a well socialized herd. They have a bit of a time at first learning the "rules" of that particular herd. I have also relied on Benevolent herd "Leaders" who are balanced and even handed to help me straighten out my little "Hoodlums" or young colts and teach them manners and rules.
I feel like if we have a good understanding of herds and the horses you are putting together AND a good amount of space for them to work it out in, It should be a relatively smooth transition with proper preparation, introductions,and leadership.