For quite a while, it seems to have been generally agreed that medicated chick starter was not a good idea for ducklings. The concern, if I recall correctly, is that ducklings consume much more feed more quickly, therefore overdosing on medication that, in a relatively healthy environment, is not necessary for them.
Since feed companies started using amprolium as the medication, many have come to the conclusion that amprolium will not harm ducklings and therefore there is no health concern feeding ducklings medicated (with amprolium) feed.
Aside from the medicated/non-medicated concern, however, is that feed formulated for chicks may contain only a third of the niacin ducklings need for optimal development. In fact, it appears that some ducklings are very susceptible to a lack of niacin, and it affects their nervous system, sometimes proving fatal.
So. The medicated concern is something you may just need to go with your gut on. The niacin, however, I feel is a big concern and I would not want to put my ducklings at risk just to find out if any of them will become sick or die if they need more than is in the feed.
If chick starter is all you can find, you can supplement niacin with capsules dissolved in water (100 to 150 mg per gallon, according to Storey's Guide to Raising Ducks), a sprinkling of brewer's yeast on their food, or other form of niacin (bugs and worms, I am told, have niacin in their bodies). If you do provide grownup food for the ducklings as many do, be sure they have had a sprinkling of chick grit (small grit or even a little sand) some days before, so they can digest the food and not become impacted.
Hope this helps.