Keeping a healthy amount of ducks (8-15 Ducks per acre as recommend in Storys guide) Can actually be beneficial to the pond's health. They can quickly kill invasive species and also provide valuable manure to the pond, which in turn helps feed the fish. At least 20% of a ducks diet includes fish/snails/worms, but I wouldn't be concerned about them overeating given you have a healthy amount of ducks per volume water. I couldn't see them eating big bass if anything they would eat some small chad or baby bass.
I would recommend feeding them either a Formulated Duck feed or an All flock Feed, Both include more niacin then ordinary layer feed, which is essential to a ducks health. They will also need some sort of shelter to be protected from the elements and predators. Does your pond have snapping turtles? They can pose a serious problem to ducks.