I don't find it unusual at all for a gelding and mares to fight. It's all part of establishing pecking order. Years ago had a gelding that had been running with the mares, and we definitely got both of his "screamers" when he was clipped. Added a new mare and he wanted to kill her. But she was not about to back down. They fought so hard that they ended up leaning against each other's rear ends so that neither could let go with a good kick. But in a couple of days all of the drama stopped. I figured I would just stay out of their way and they took care of it themselves.