Of course they can breed with one another :P The babies will just not be able to produce eggs and the males will be sterile.
I have seen my dog try to breed with my cat - so anything is possible :gig
ETA: Oh and there is such a thing as a turkey x chicken and a guinea x peafowl, so there is...
That is also what I've heard. I have also read that if the male is muscovy and the female is mallard derived, the offspring will be sterile. I have not heard what would happen if the other way around, though I'm sure it's the same.
They CAN have babies - if one is pure muscovy and the other...