Q: Can different waterfowl breeds cross-breed?
A: Yes, it is genetically possible for any breed of duck to cross with any other breed of duck, and any goose breed can also cross with other breeds of geese. We are not aware of any verified, viable crosses between ducks and geese, however. Sometimes a goose will attempt to mate with a duck, or vice-versa, but even if they mate successfully, the resulting eggs would not be fertile.
Each breed will likely prefer to mate with other breeds of similar size, but not always. It can be difficult for a larger bird to catch a smaller breed or for large breeds--which mate more successfully in water--to mate with other breeds on land. Sometimes, however, they are successful.
When breeds cross, interesting things can happen. One example is the prodution of "mule" ducks (the result of crossing Muscovies and Pekins), which we discuss here. If you would like to breed pure-bred ducks and geese, it will be important that you keep each breed separated in their own breeding pens.