From what I've read, peat (or soil) is a mess waiting to happen every time you move plants. Pea gravel is so much bigger than fluorite it will end up on top before very long. Mixing different sized substrates (ie sand & anything) leaves a less than pretty mess. If you really want to try it, at least use the same color. White sand (or pale pea gravel) & dark, even small gravel, looks bad IMO.