I find the easiest way to add niacin for ducklings is to buy Brewer's Yeast and adding a tablespoon over the chick starter every day. It won't hurt the chicks, but if you are brooding them separately, you can just add it to the ducklings's feeder.
If you are going to give treats other than soft things like boiled egg yolk or cottage cheese, they will need access to grit. If you don't give them treats until they are outside, they should be able to pick up small pebble bits to use as grit.
I don't give scratch to my flock; it's too high in carbs. I do treat with Black Oil Sunflower Seed (BOSS), which is higher in protein than scratch.
If you are using medicated chick starter (I do) then you continue to feed that to them for at least two, if not three weeks, after they are living outside. Then I change over to an "All Flock" feed, as I have chickens, ducks, geese and turkeys. Once they are on All Flock, they won't need the niacin supplement. But they need the amprolium in the medicated feed WHEN they are exposed to soil or the med won't help them build immunity to coccidosis.
With an All Flock feed, you will need to provide calcium free choice in a separate bowl or container ONCE they are starting to lay.
Good luck!
