The frogfish is best a in species tank, they will actually eat anything up to their own size, so in a smaller tank, that task in itself is difficult. They will be fine with hard bodied inverts although I recommend against crabs(maybe hermits), just in case they nip at them(no emeralds, etc). I've found snails and hermits work fine, urchins as well but be mindful of the urchin getting enough to eat, especially difficult in a small tank unless you target feed. When they do eat something as larger as themselves, both die, so don't risk it.
Don't fall into the "train your frogfish" trap, these guys do not live long on a dead only diet. Their nutritional needs are never met with a dead only diet(maybe 1 in a 1000, I just haven;t met anyone). Ghosties will likely be your staple, but an appropriate sized guppy or molly will also be a good idea. The live gut flora in the fish is very beneficial. Little guys can be ok to feed every 2-3 days at first then eventually work into 7-10 days. They are rather sedentary creatures and will get more active when hungry, eventually you'll learn his schedule for feeding. These guys live on a gorge/fast feeding routine, so feed them their full then give time in between for digestion. Infrequent big meals(not big items) is better than frequent small meals. Feed until you see a little bulge in his belly. Maintaining reef level of mg helps with digestion.