there are no rules. Do what you like, you're the one living in it.

(not a pro) Post some pictures of your counters with tile samples propped up against the wall, from up close and further back. Personally I think marble and granite, while looking great separately, may not look great together. The patterns may fight each other, unless your granite is a solid or near-solid color (like absolute black.) Even then, might be too busy.

Probably not. But we need to see a photo as Chicagoans mentions above.