When I bought my horse, he was only 10months at the time, he was turned out into a pasture with about 8-9 horses as well. He got bullied quite a bit, especially by the lead gelding, and the lead mare. He'd come in with bite marks, hair missing, kick marks, new wounds every day for about a month or so. Soon after though, he started to get "in" and was best buds with not only the lead gelding, but the lead mare as well.
I know it can be hard, I was beyond nervous about letting him out there, but they do seem to work it out