Ducklings are a lot of work! They love to make messes and they poo even more.
What to put them in.
They can be in anything for as long at it has high sides. Kiddie pools, tubber ware containers (the large ones for clothes), the bath tub, guinea pig cages, ect. You want something with higher sides because ducks can mange poo out at sonic speeds which somehow gets the poo way up on the side or even up and out of the cage which no one wants to have projectile poo everywhere!
Water and feed.
When a duckling is hatched from an incubator they lack the necessary oils that a mother duck would be able to give them that would allow them to float in water. With that they should never be given enough water they could drown in. A small plastic water containers that have the dome you fill and twist on the bottom with the slots over the water ring would be adequate and allow them to drink but not crawl into it or a shallow lid from food containers. You may need to show them where the water is by dabbing their beak in the water as well as the food. Never give a duckling feed with out water. They need the water to properly swallow and can choke and die with out it. The feed you should give to ducklings can NOT be medicated. You can give them the Poultry Starter by Dumor which is sold at Tractor Supply. Ducklings need more niacin then what most feed contains so you can give them electrolyte in their water which is also sold at Tractor Supply. I stop feeding the Starter when their feathers start growing in, then you can start the Finisher and after you use the bag up you can start to give them regular adult feed. They need niacin and giving them it directly in the water from brewers yeast will work or the niacin tabs you can find at a local store that carries vitamins should have them. Giving them peas is also a good source of niacin and they love them along with finely chopped lettuce. The should not be fed worms until they can be wormed and properly treated because worms can carry Gape-worm which is a nasty parasite. Ducklings can be put in a tub or other water holding device to "swim" with supervision but don't allow them in too cold of water because they cramp up and can drown. Make the water luke warm so it's not too hot either.
Temperature and bedding.
They will need a heat lamp for the first 3 weeks. The brooder should be 95 degrees first week, 85-90 degrees the second and 75-80 the third. If it is cold outside then hold off on them going out until they feather out. They will let you know when they are too hot or too cold. When they are far from the lamp and panting they are too hot and the lamp can be pulled up higher to make it cooler. When they are huddled as close as possible to the lamp and to each other they are too cold and the lamp could be lowered a bit but be careful that if you have a really strong/hot light to make sure you don't have it too close to anything that can catch on fire or melt. Do not lower it so far that the ducklings can bump into it and burn themselves. At all times make sure the lamp is SECURE there is countless stories of people losing an entire barn over a lamp that fell and caught bedding or something flammable on fire. Do not think it can't happen to you. Bedding will get wet constantly and if it is not changed enough it smells terrible when you do change it. I have used flake bedding not the 'fine' or sawdust like bedding because the ducklings can eat it and can lodge in their throats. Stay away from cedar bedding. There are reports of the ducks having an allergic reaction to it or coughing. Puppy training pads work for the ducklings who like to find ways to eat the bedding and can easily be pulled up and allow for easy cleaning.
When to let them with the others.
I would let them with the others once they have get used to the idea of being outside and are familiar with the others with a fence between them. That way the adults do not think they are complete strangers and want to attack them or beat them up. There will be slight poking a chasing and it is the normal pecking order being shown. Just keep an eye out to make sure no major bulling is going on.
Ducklings do grow up and will no longer be "small, cute, and fluffy" and can NOT be dropped off at a local pond! Most ducks are domesticated and will not make it threw winter. And no their "natural instincts" will not kick in like a wild duck breed will. Just remember the saying "like a sitting duck" because that is exactly what your once "small, cute, and fluffy" duckling will be for any predator that is in the area.