Ducks are waaay different from chicks in their needs. I picked up 2 baby ducks at a feed store after the proprietor told me I could keep them with my chicks, but it didn't work out that way at all. Unlike chicks, ducklings need constant attention from "mom", which you will be if you get ducklings, and will cry heartbreakingly unless they are with you constantly. They have absolutely no interest in chicks and will follow you around endlessly. We didn't have the fortitude to let them suffer, so we gave in and kept them next to us as much as possible. One of our ducklings died very quickly, but the other survived and became our family "dog" until a possum got it one night (she was trying to get back into the house with us, and escaped from her outdoor cage). My kids took our duck for walks on a leash to the frozen yogurt store, etc. It's really like having a puppy around - they are amazingly cute but need a lot of attention. A year later I still think about getting another one - in the house, as a family pet, with diapers - but I always decide not to do it because of the insane amount of care involved in keeping a duck indoors. Their ability to poop cannot really be underestimated. We clocked ours by feeding her blueberries and watching for the unmistakable end result - I kid you not, it was slightly over 2 minutes from entry to exit.
As far as nuts and bolts: baby ducks eat duck starter (which looks like chick starter but I guess is a different formula). They drink from a dish, not a nipple waterer, and make a fantastic mess when they do. They will carry their crumbles and drop them in the water to moisten them, then spray the resulting slurry in all directions as they suck it up. Very funny when they are small, but a huge mess when they get bigger. I spent a lot of creative energy trying to figure out how to keep food and water separate. In short I think you will have trouble keeping ducklings together in a brooder with chicks both for the emotional health of the ducks, and for food and water issues.
Good luck!