when I had one duckling with my chicks last year i fed all grower for chicks (unmedicated, of course!). I did watch the duckling in case she had an issue, and I had nutritional yeast I sprinkled on her food. I would've gotten brewer's yeast to add if she showed any problems, but she never did. I had her outside pretty early anyway since she was a July hatch, and she caught bugs to make up her own niacin I think...
Nutritional yeast is higher in niacin than brewers yeast.
Its what i use too