As long as all the material is recommended for aquarium use, you can mix it. In time, with all the water changes and such, it will be a little mixed anyway. The easy to keep plants you're going to have can be attached to a piece of driftwood or lava rock with dark sewing thread and don't require putting into the substrate. They'll grow fine just sitting on top.
"Fear not, my young apprentice. Just change the tank water."