My fear is that those vitamins are too potent for tiny little ducklings as they were made for giant humans, regardless of the water-soluable trait. This is because most vitamins are chemicals, not foods like herbs are which makes them safe in their unprocessed form.
If you continue to mix your own grains, think about adding cracked/crushed wheat. It is highly nutritious. I have personally always used a chick or poultry starter and supplemented with free range, scraps, etc because I was always worried about deficiencies and poor growth.
I think you should...