Elephant nose fish can live bichirs or any other fish for the most part, you dont want to put them with fast eating fish because they will steal all the food from the elephant nose fish. A school of 6 or more neon tetras would be fine they eat fast but due to their tiny mouths and bodies they dont steal much food and they school slowly. You can also put it with a african leaf fish as well. Really anything that eats slowly and wont harass the elephant nose fish and wont get harassed.
But with the size of your tank I wouldnt put to many fish, might wanna mix their adult sizes together and figure hat out relative to your tank. Rule usually is 1 inch adult fish for 1 gallon, you always need to follow this if you have good filtration and do lots of water changes.
