It's funny you say that, because thats exactly what I did! I used a netted window screen to separate them. The big one seems to be coming pretty close to the fence. Maybe becasue he want to get close to them?Separate them with a wire fence so they can see each other, but the older one can't get to the younger ones. The size/age difference is too great if there's roughly more than a 2 week difference, or when one group is a week or younger (as they're still finding their footing and can easily be run over). When the younger ones get a bit bigger and can stand up to the older one, you can try removing the divider and see how they behave around one another.