What kind of geese do you have that would kill each other? Wow. I have 3 Embden hens, a Toulouse hen and a Toulouse gosling that my Toulouse hen is adopting (yay). They all live with Runner ducks and are fine. Two of the Runner drakes have claimed one of the Embden hens and they "mate" with her, silly ducks. I think their compatibility has very much to do with the fact that they roam over an acre all day long and have 2 play pools and a 6ft. round stock tank to swim in. They are not crowded, they have plenty of water and food and they have separate night pens. If they were stuck in some small dirt pen with just one play pool, as I see often with farm birds, they would be so unhappy that their frustrations would rule their behavior and I would probably have some dead ducks, and sick birds. I think that if your birds have plenty of clean space, grazing, food and water they should get along fine with each other.