I approach things a little differently. I have 2 groups, one of 5 and one of 3. The group of 5 is 1 gelding and 4 mares, the group of 3 is one stallion, 1 gelding and 1 intact but soon to be gelded colt. They get along great and if I were to add someone to either group, it's up to them to get along. If a mare went into the larger group and was getting picked on, after a reasonable amount of time, if it didn't settle down, I would sell the newest mare. If I moved a gelding into the smaller herd, I'd expect a few fusses but I'd also expect him to find his place fairly quickly. If he couldn't, then I'd move him on.