A couple of ducks will probably fit in with you chickens just fine. They need about the same amount of room and, so long as they have a little water to bath and splash (and breed) in they are happy. You may even be able to house them in the same coop. I know people who do, I don't. But, I have 24 ducks and the same amount of chickens so I would need a really large coop. Mine all have their own coop and run but share the same free range. I do have two distinct flocks of ducks which I am having the keep apart since the older males are trying to kill the younger males. Hopefully that will pass.
But back to you and ducks. One or two (preferably two) should do fine with you chickens. Try getting two females unless you really want to breed. Ducks tend to be messy since they seem to have to put everything in their water so you will need separate water for each. They can eat the same food if you use a mixed poultry food like Purina Flock Raiser once they are adults.
Good luck.