FW ghosties (Paleomonetes sp.) and "grass shrimp" (Palaemonetes pugio) are basically the same...it's where they're collected from. "FW" ghosties are collected from the estuaries where the salinity can be very low, however, they are also found in SW grassbeds off the Gulf Coast. You can actually slowly acclimate them to SW if you're so inclined.
However, if you're using them as a food item, all you really need to do is enrich them by feeding them a good quality marine flake food, spirulina, beta glucan, etc and feed them out. Whenever we get new scorps in that need to be conditioned prior to weaning them, we keep a critter keeper with a small HOB filter (such as a Palm Filter) running for them.
HTH
