Chicken feed is normally to be too low in vitamins and sometimes too low in protein for ducklings and goslings, you can give it to them for 5 days and it will worm them but definitly supplement the feed with a vitamin solution as well, particularly if it has multiple B vitamins, E and A are also very important. Adding nutritional yeast is another option.
20% to 22% protein is typical for a feed, anything lower isn’t enough and you’ll need to supplement with something to boost their protein like soldierfly larva or meal worms.
Antibiotics are illegal to use without a prescription in my state also (California) there is a loop hole though. Fish antibiotics are repackaged antibiotics manufactured for human use, they don’t require a prescription, neither does antibiotics marketed for pigeons. I have no idea if it’s the same in Ireland, if you can even find them or not. Just adding that on the off chance it helps.