I am not positive, but I think gapes are more common in ducks than they are in chickens, but don't quote me one that. Tubing, casportpony taught me all I know, lol.
There is no downside to worming, so I would definitely go ahead and worm all of them, or at least just the two. Bigger duck will need more wormer, so let me know if you need help figuring out the dose.
You are correct... one of the methods I use to stop them laying eggs, like you would want to with an egg bound one, is to put them on a very strict diet.