No organic material is needed in the sand. I use thicker 4 x 8 sheets of styrofoam. It is durable and easily cut to the right height and to make the inlet dips. Its fault is that it that it floats. To keep sand from filtering into the pond and yet let the water seep in, place a log across the openings in the retaining wall but on the bog side of the wall.
Most species seem to adapt.
Then ROLL up the green moss. They will fill in fairly quickly. Pitcher plants - Sarracenia species esp. Venus's Flytrap - Dionaea has wintered 5 winters for us in USDA zone 5. All orchids in Michigan are protected from collection without permit by state law.