My trainer always turned out her gelding and mare together with no issues really. When she bought her new property, she ended up with her gelding, her mare, my mare and 2 pony mares. She slowly introduced everyone, for a while we had her gelding and mare together and my mare with the 2 ponies. Then she went to big horses together and ponies together, now all five share a 15 acre turnout.
So, 4 mares and one gelding turned out together. It's a hilarious herd dynamic we've got going on. We have my mare who is a bottom of the totem pole horse, her idea of herd survival is to go far away from everyone else and you won't get beat up. Well, my trainers mare loves her, so she follows her everywhere and the gelding is afraid he'll might get left out of something good so he follows along too. Usually he'll be busy grazing, notice *his* girls have left and be like hey, wait up!! and go chasing after. Then there's the two pony mares, they just follow along behind everyone else unless people come to the pasture, then the big horses stand there going duh..... and the ponies RUN for the people because people bring treats!
I don't think we have a single dominant horse in that group. We have no squabbling, fighting or marks left on anyone. It's like a giant group of Lemmings, all happily following the stupidest one. Oh look, she's in the pond, me too, me too, oh ooops I just lost a shoe. Oh look, she's eating an entire tree of apples, me too, me too, oh dang, now I have a tummy ache. Oh look, she doesn't come when the humans call, now we don't either!