I knew there was a reason. :hugs
I didn't think the duck would be on the balcony - I just mentioned it in my reply for context. The duckling will need to be in a brooder until fully feathered. So, you're doing the right thing as far as the set up. It sounds like you've thought this out and have...
I don't want to dissuade you from getting another duck and generally I'm all for it because I believe they should have a flock. However, in your situation it sounds like it's not an optimal environment for even one duck since you mentioned it would be on the balcony.
We've seen a similar...
You can feed the chick crumb or even an all flock feed. Most all flocks are intended for those with mixed flocks including ducks.
As for supplementing, it's best to supplement all newly hatched ducklings with niacin. Usually if there aren't significant problems, nutritional yeast is...