I had my ducklings on starter for two weeks, then over the course of the next week, added more and more grower to the starter so they were on grower by 3 weeks of age. Around 8 weeks, I think we started maintenance rations, and then at 16 weeks four days someone started laying and we went to layer.
Hard to estimate how much water, there is some variability depending on the ducks, the weather, the waterer, etc.
I went to a five gallon poultry waterer when my ducks were maybe six weeks old. Then once outside, we went to open pots, about two gallon capacity, and swim pans.
The ducklings also, by about week 2, had head washers to prevent sinus, eye and ear infections, and we went for a bath - no soap - once a day in warm water up to the belly. Only lasted 5 or 10 minutes, but it cleaned them up - they had crumbles (and then the little pieces of the straws that wrap around new feathers) in their feathers, so I got them in the tub daily.
I would not leave ducklings alone with chickens - the latter could go after the ducklings. I would use some plastic poultry fence or similar to separate them. And wouldn't the chickens hate all the water splashing? How old are the ducklings? I let mine have water and food 24/7 for their first 8 weeks, then only took it away overnight. They grow so very fast.
Once the ducks are adults, no drakes with the chickens - equipment mismatch could lead to injury or death to a hen, I am told.
Why all the postings about sick ducklings? My sense of things is that, in spite of ourselves, when we first start out with ducklings, we don't always get everything right. We may not know that many ducklings need much more niacin than is contained in chick starter. Or we miscalculate the brooder temperature, or we don't realize ducklings need a head washer so nares get plugged up, or the duckling swallows some bedding. Most often it is those kinds of things, seems to me. Ducklings can get dehydrated in transit, or not quite perfectly tended at the feed store. Several people on the forum have brought home ducklings because they were obviously neglected or imperfect. Those are my thoughts. Nothing official.