Do not feed ducks medicated chick starter. They eat more and can overdose. The ducks need waterfowl starter if its available in your area for the niacin. If its not available nonmedicated chick starter along with a niacin supplement is the way to go.
Our baby ducks are eating chic starter. That is what we were told to feed them. They are not with our chickens yet. They are only 3 weeks old and our chickens are much bigger. Also we have 18 new baby chics (just got them today) so they are to liitle to be with the ducks. Eventualy we will have them all together.
I purchased some chicks 2 wks ago, days later a neighbor gave me a bunch more. All in the same brooder. Purchased two small ducks at TSC. All are doing fine in the same brooder. But boy do those ducks grow fast. Chicks are getting their feathers in. Will be three weeks soon. Though all have grown. The ducks outgrew the chickens already. When I got them they were about the same size as my smallest chicks. Now they are double their size. All they get is what the chicks get. Chick starter. And plenty of it. As well as a dip in my pyrex from time to time. They love it. All are thriving!