It definitely depends on both the stallion and the gelding. Our stallion can be put with most geldings, but if you have a very dominate gelding who constantly challenges the stallion, or if you have a gelding who is proud cut, or cut late, and still thinks he's a
"man" then you may have problems. I would try it across the fence first and see how they react to each other. If that is good, then have a trial with you there available to seperate them again if necessary. Good luck.