My barn has one stallion. He used to be kept away from the mares when outside, but now his paddock is down the same path as their's. He's still two paddocks down, though, they're not directly across from him.
He respects fences VERY well, and the fencing is strudy and quite high, so there's no worries about him getting out. Heck, his previous pen was just 2 strands of hot wire and he never got out of that. This other fencing is like, 7 or 8 strands of wire (not hot).
He isn't kept with geldings or anything, but there aren't any issues with him at all. He is very well mannered and behaved.