Waterfowl are fine on a all purpose feed like Flock Raiser. When your ducks start laying you can offer oyster shells for extra calcium. You'll want to check out your city's ordinances though because most will not allow for free ranged fowl. Silkies will be fine hatching and raising ducklings but the inverse does not work. Mother ducks taking a chick swimming can be fatal since they were not designed for water.