Koi is just an extension of the Marble gene, much like Grizzle. So he is technically carrying the Koi gene but selective breeding will give the "koi" look which is usually like a fish with a solid under color with some spots over top like a dalmatian but bigger spots.
As for breeding marbles, I had heard mixed reviews as well. Usually I guess you get mostly cambodian's and solids in F1 with maybe some marbles showing up, they're all carrying the marble gene's but most just won't show it I believe. And then come F2 you start to see more of what you're looking for and then just keep inbreeding till F4-F6 until you get what you want basically.
I believe that's how it goes, feel free to correct me.