All Flock crumbles for all the birds is perfectly acceptable. Ducks need extra niacin, which may or may not be present in chick starter. Sadly, US labels don't have to disclose niacin levels, so even if you see it on the ingredients list, there's no way to know if its at chicken levels, or the much higher duck levels. Nutritional Yeast is a common, off the shelf, way of supplimenting Niacin levels.
BYC posters have previously reached out to both Purina and Nutrena regarding Niacin levels in their All Flock/Flock Raiser products, and the numbers offered by those companies were acceptable for ducks at that time. I can't speak to any other feed, or if Purina and/or Nutrena have subsequently reduced niacin levels. Its doubtful, but I just don't know.
Niacin overdose levels are MUCH higher than duck levels - so no concerns with chickens eating "duck feed" levels.