With geldings, (I assume there are no studs? It wouldn't make a whole lot of difference if they are mild mannered, but still something to consider,) I don't think there is a whole lot to worry about. I have one alpha gelding and then the rest of my herd is mares. I introduced a very submissive gelding into my herd and the first few weeks were rough because the new gelding wanted nothing more than to follow the herd around. Most of the time a horse would just give him a dirty look, but my gelding would chase him all of the way across the pasture and leave him there. So for the first few weeks the new gelding spent all of his time way in the bottom of the pasture. He then started to bond with another Paint, who used to be my gelding's 'favorite' mare. After a while he warmed up to my gelding's newest favorite who is a quite a bit younger, and now him and my gelding are best of friends.
I would wager that they'll probably stay apart for quite a while, but sooner or later you'll notice the other ones wanting leadership from the gelding who keeps chasing them away, unless there is a more dominant one in their herd. Might be some cuts and bruises from learning how big their 'personal bubbles' are, but other than that in the end it will probably end up as a good thing.
As an added note, every time I add a new horse to my herd it usually takes quite a while for things to level out. Most it has ever taken was a few months with my 3rd horse. But since her it only takes a few weeks and they are all good.