Agree w/ everyone else, it's all individual and has nothing to do w/ gender. Currently my dad's gelding is staying w/ my mare, she gets along w/ everyone. The gelding is more dominate, hard to tell w/ just the two, but you can tell at feeding time (very mild, sometimes they share).
Some horses are just ill tempered w/ everyone, but over all I've had more issues w/ geldings causing problems. My sister and brother in law's geldings have lived together fine, then we put our pony mare w/ them, and the fight was on, over the mare, not involving her, she gets along w/ everyone. They've settled down now, but there for awhile poor Wizard was shunned, and Genie couldn't move two steps w/out Magic stuck up her butt.
When my dad's horse visits he stays w/ Gypsy because I know they'll be find together, and I'm pretty sure Magic (gelding) would fight w/ him.