Rarity does make a great fairest, but if your going for one pony per line, that means someone has to be a vampire, and I don't think that Applejack is more Vampire than Rarity.

As for the Wolves, I just think that the Apple Family angle makes them the more obvious wolves, as the whole pack thing should be a significant theme.

I know next to nothing about geist. But what does vampire mixing well with geist have to do with anything? Applejack and Pinkie do so very little together.