Well, judging from Nick's posts I would say fluorite is a fine substrate to use. Blue diamonds--although they look amazing--are still just Neos. Which means they live in Neo params; you won't need to buffer the water down to 6.0 PH so long as you control your kH and gH.
How thick of a substrate layer are you planning on having? I have an unopened bag of amazonia that I would be fine with sending you some of. I don't think you want to mix substrates, but you could always have a flat rate box of amazonia underneath your fluorite to help buffer just a little bit. The small amount of amazonia would also leech ammonia to help with your early cycle.