Shouldn't be a problem. Goslings can take care of themselves pretty well because they are like little snapping turtles, pinching everything that comes near them. That will be beneficial because ducks are so clumsy they will learn to give it a little space.
None at all. It's exactly the same. Their behavior is a bit different though. Ducks are a little more lively and mischievous. Geese are more reserved and territorial. The geese take more of an in charge roll of making sure everyone is behaving and safe. The ducks will act pretty well like a bunch of hooligans.
gosling may be a bit larger that ducklings when they hatch but they are still pretty fragile and should never be put with older ducklings they need to be treated just as ducklings with brooder/heat and all the tings that go with it. 11.5 week old ducklings could really put a hurt on a 2-3 day old gosling,