@ Dan: I think that's a bit of a stretch. Not all Uchiha have a natural fire affinity, but as it was said earlier anyone can learn any nature manipulation with enough effort, it's just that people who are using their natural element will have a much easier time and thus can focus more on refinement. I'm about 95% confident that Sasuke's natural element was lightning and he leanred the fire skills because they we're taught to him.

As to Kazuku, it's possible that the hearts he's stolen include the techniques those ninja knew, not just their elemental affinites. So if he had taken Sasuke's heart he would be able to use Chidori and Fireball technique for example.