Antennarius maculatus is probably the only common frogfish you're going to find that will stay small enough for a tank that size. Pretty expensive, but gorgeous.
You'll probably have to start off with live food, shore shrimp. Don't feed freshwater stuff if at all possible to avoid. Stick silversides on the end of a stick and wiggle it around in front of the frogfish, and it will eventually get the idea. You don't need to feed them everyday. They will die if overfed too often.
As for care, just provide good water quality and not too much flow. They aren't good swimmers and they will die if they are always fighting the current to stay in a spot they like.