They are two completely different types of substrate. I wouldn't think there would be a problem with mixing them except the Fluval is so much lighter I would expect it to migrate to the top and end up as a cap. In simple terms the Fluval is compressed soil and the EC is rock. Fluval is an active substrate that provides nutrition for the plants. EC is basically inert. Thorough rinsing of the EC will get rid of the dust. Thorough rinsing of the Fluval will get rid of the entire substrate. You can rinse away all the good stuff you're paying for.