Yes the biggest thing ducklings will need is niacin. What I bought for my ducks that is AWESOME for them is Mazuri Waterfowl Starter Diet. Has pretty much everything they need and they enjoy it too!
Once they were fully grown I switched their feed but still mix that Mazuri stuff in there just...
Yes ducks can eat chick starter. Ducklings really are technically called chicks so just like they eat chick grit. Just make sure the chick starter you have has all the nutrient they'll need in it