Are there any duck species that will only land with their own kind? In other words, are there any ducks that require species-specific decoys, or would a spread of mallard decoys work for everything?

I've seen them all get that way if they're under a lot of recent shooting pressure. Also, if there are lots of ducks or decoys crowded in the area in which they're hoping to land, they will show a preference to those of their own species. More so than others, I've seen pintails, black ducks, and widgeon decoy much more consistently to spreads of their own kind - having said that, if you're looking at pintails, avoid hen decoys and concentrate on filling the spread with males - that's what they see and concentrate on. There's not not much difference between a hen pintail, hen widgeon, or hen mallard. On the other side of the issue, greenwings and bluebills will at least buzz once any species of ducks on the water. My experience was mostly on the lower end of the Atlantic flyway, where ducks had run the gauntlet and been hammered to hell by the time they got down to us, so things may be entirely different in your part of the world.

Puddlers it doesn't matter.

Divers, the pattern will matter, but not the constituents.

Go forth.