You can, but you want to be very careful to not unbalance their diet.
If you are feeding commercial feed, it should be balanced, so you don't need to add vitamins or minerals. If your birds are foraging and you've got good forage and lots of bugs, the birds will balance their own diet.
Crushed niacin is added to the water of ducklings because regular poultry food for chickens doesn't contain enough. But if you buy waterfowl feed or gamebird feed, it contains enough niacin for ducklings.
Calcium is best fed by giving free choice oyster shell. Then the birds will get what they need and not more than they need.
I would shy away from the garlic. Garlic and onions can act as blood thinners. Great for people but not so good for livestock.