Congratulations on your new ducklings! You will have a great time watching Duckling TV.
I would recommend reading, reading, reading and would get the book Storey's Guide to Raising Ducks by Dave Holderread and the one from Hobby Farm too. Just a couple of all around good knowledge books. This website has been indespensible for me with my ducks and chickens. Lots of very helpful people and lots of information.
Because my chickens and ducks grew up together, I was able to buy just one type of feed from when they were chicks/duckling up until adulthood. Because you have ducklings, you will need specific feed for them. I presume your chickens are layers and are on layer pellets. Once your ducks are that age, they should be able to eat the same food. Eating the same treats is ok too.
Ducks like to swim but others even prefer to wade in shallow water. If you don't live near a body of water, a large plastic tub or kiddie pool will be fine.
Laying varies but between 4-6 months.
As for your building size, perhaps 3 (4 at the most). Though I like ducks, I'm glad I got only 2. They are quite messy, as everyone will attest to. You want to make sure they have room and not all squished together.
Good luck!