Medicated chick starter will not harm your ducklings. It only has Amprolium in it, which is a thiamine blocker to restrict the growth of coccidia and allow the chick or duckling to develop resistance to the parasite. I've raised ducks on medicated chick starter, adding Brewer's yeast a couple of times a week. As a matter of fact, I'm raising 3 call ducklings I hatched from eggs, right now. They're doing fine.
If you can get "meat bird" starter feed, that's best for them, though.
Ducklings grow very fast. Unlike chicks, they put all their energy into building their body bulk first, instead of feathers. They'll stay fuzzy for a long time, but grow bigger FAST. (Unlike chicks, which put their growing energy into feathering out.)
It's also important you provide a water source deep enough for them to put their whole bills into, so they can clear their nares (nostrils). What I use is a cottage cheese container, with an oval cut out of the lid, then put back on it. That way the ducklings can put their heads into the container but not get INTO the water (which they will want to do, trust me). They will fling water all over their brooder, too. And when they eat, they have food in their bills, when they put them into the water, so the container will get murky with the wet feed really fast.
