20 gallon won't be big enough as they enjoy swimming around on the bottom of the tank. I believe you need at least a 4' footprint for them. Groups are debatable as once they get older they just tend to fight for caves and hiding spot. You could do a trio of 3. I myself have 2 and they are never with each other, they just fight over who gets the better cave. Also, keep in mind they are nocturnal so you won't see them out a whole lot during the day. But, they are still pretty cool!!