Separating mares from geldings will not necessarily prevent fighting. The social structure of a group of horses is based on an established hierarchy, with some animals being dominant, and some submissive, regardless of gender. Separating individuals that do not get along and don't seem to be able to settle into a stable structure could decrease the conflict in the herd, but separation based solely on gender will not solve the problem unless by chance you separate the horses that are creating the conflict.