Mixed feelings about this, I have used chick starter medicated with no problems. It even talks about medicated in Storey's Guide about using it (with other options too) which is like the bible for duck raising. I use the starter feed and now on the flock raiser and will start using layer feed...
From what i read the major difference in ducks and chickens is the Niacin. My ducks after i bought a bad batch, I started giving niacin to them in their drinking water. The feed i started to use is Purina starter for my ducklings.