I raise runner ducklings with Sebastopol goslings that hatch in an incubator in the spring. They are best buddies for a few months until the ducks reach maturity. At that time, I separate them.
They might all live together happily ever after on Old Macdonald's farm, but on my farm, serious...
Niacin...give the duck brewers yeast, peas, and/or a duck specific feed like Purina. If you can get some Niacin vitamins in little capsules, you can sprinkle the powder on some defrosted frozen peas for a niacin boost. This is what I use for my young runner ducks: