You can really put any brackish fish in with them, a trick to keeping a figure eight green spotted red eye topaz puffer ect from being so aggressive and outgrowing its environment is to feed them freeze dried only, never feed them live or frozen food, they will grow fast, and rather ornery towards other.....I suggest for a twenty high with one figure eight ....two rhino gobies and a dragon goby, perhaps add a knight goby as well...or maybe a jade boby and a mono or two...stay away from mollies they tend to spawn all over your tank making it a mess...if you choose a jade goby rhino gobies or a dragon goby be sure to have a soft substrate cause that's where they will be...plus with sand a dragon goby is a blast to watch eat and a master at cleaning your sand