Andrew, as long as the quarry tiles are unglazed, they are food safe. If you're really worried about it, then, sure, get the fibrament. Fibrament will give you similar results to quarry, though, at probably 6 times the cost.
Insulating firebrick is about as heavy as styrofoam- it has very little mass. The probe itself will be more than strong enough to hold the brick. You will want to trim the brick down so it's not a full size brick, though- I'm thinking a 1" x 1" x 6" with a hole drilled down the center for the probe. It cuts/drills much like styrofoam as well- very easily.
Your fibrament hearth should be on the bottom shelf of the oven. When you have sufficient top heat (such as a broiler), thicker hearths are usually better, but, in this instance, the goal is to handicap the hearth so the ceiling can catch up. 1/2" fibrament @650 (your target temp) is kind of uncharted territory- it might be a tiny bit too handicapped, but, imo, it's better to have too weak of a hearth (and need to push to the oven to 675) then to have a hearth with too high heat transfer and have to pull the temp down to 625. A quarry tile ceiling @ 625, even just an inch or two away from the top of the pizza, will be abysmal.
Remember size matters (No, that's NOT what she said
), so get the largest stone your oven will fit. It looks like Fibrament has a 17 1/2" x 17 3/4" x 1/2" stone. That should be a very comfortable target for a 16" pie and slightly less comfortable target for a 17" one. If you're lucky enough to have an oven that can handle an 18 x 18 or larger stone, then get the 20 3/4" x 20 3/4" stone and have them cut it down. I really hope this stone makes it to you in one piece. Fibraments are notoriously fragile and, when you get into thin and wide dimensions, flexural strength plummets.
These are the kind of temperatures you're oven will be hitting.