Sounds like you're doing just fine. Don't let the worries freak you out. It is true that ducklings are different from chicks, as you've discovered through different growth rates and the water issue, but it doesn't mean they have to be complicated or that you can't raise them together.
The only thing I would add to the discussion is to keep your eye open for potential niacin deficiency. Ducks do thrive on higher levels of niacin, but that doesn't mean you can't raise them without extra. If you had a whole batch, or were planning to raise more than one, or if she were separate from the chicks, I'd recommend that you go ahead and start adding niacin. But given your situation, I'm just going to say that you should keep your eye out for the symptoms of niacin deficiency, and be prepared to supplement if she develops them (and she may not--many never do). If caught early, niacin deficiency is easily corrected by adding the right amount of niacin to the diet. And the symptoms are consistent and easy to spot: weak, trembly legs. That's it. If you see weak, trembly legs in your duckling (and not in the chicks), you may have a niacin deficiency (though there can be other causes) and should come back here and ask for directions for supplementing her feed.
Good luck. I always figure, if an animal (including my human babies) is visibly thriving, happy and healthy, then I'm doing it right. If not, then I need to address the problem. Knowledge is important, but it's also possible to freak yourself out over worries about things that never happen. Relax, enjoy, keep your eyes open for the most likely potential problems, and have fun.
