I'm no expert, just got three ducks that are about 5 weeks old now, 2 blue swedes and 1 mallard. We started giving them lettuce blend from those bags of greens from the store, at about 2 weeks old or so. They LOVED it! We are still feeding them 'meatbird' food from the farm store. Haven't given them any grit YET, but I would imagine we will soon enough.
Just like any animal as well as humans, fresh living foods is vital to good health! Greens, grass, bugs, fruit....even scrambled eggs. I think ducks are a lot like chickens, they'll eat most anything. But my chickens lay better eggs and more of them when they get greens, oatmeal and other REAL food every day.