My parents have a ranch, and we basically have two herds of horses that are usually kept separate - three young ones and three older ones. In the older herd, I have a 16 yr old gelding who is the boss, and very dominant.
Last summer the two herds ended up in the same pasture somehow (I think some gates were opened by tricky horses - can't remember the details). My old gelding ran my 2-year-old gelding relentlessly around the pasture, and nearly put him through the fence. Fortunately, my dad realized what had happened, and got everything sorted out. This was in a 40 acre + pasture.
I don't want to make you paranoid, but **** can happen. Keep an eye on them. Remove anything they can hurt themselves on, if you can. Maybe try removing the dominant horse until the others establish their pecking order.
And let me know how that goes, because at some point I'm going to have to integrate our herds (sort of hoping that once my colt is a bit older, he'll be able to stand up to the old tyrant a little better.)