I. Love. Preacher. So there you go. A Faith/Buffy dynamic wild be amazing if you have her as a slayer. She will be young, though, hop you don't mind! She'll be the free-wheeling, happy-go-lucky type, wild child-type. But with a gun. So I imagine even if they fight constantly, they'd make great friends and an even better team.
For Jo--I'm not sure about this girl's thoughts about gifted humans. She definitely finds them interesting, but she definitely is sort of a...witches-only find of girl. And Eliza can always come to her with advice about a target. For a price, of course!