Instead of just mares, ask about the horse in general. Take less dominant or destructive ones. Also, concentrate on finding boarders that you like and be sure you get references from the current boarding facility (even if they are at odds.. get their side of the story) farrier and/or vet.
I have 22 boarders and they are all mixed... I have mares that would kill each other if put together. I try to put horses together that get along. OR instead of grouping them by gender.. horses in the same age group are likely to get along better. Young horses with young horses and old horses with old horses.