like a normal 10 gallon tank or a biocube thingy?
if I were you, I would get like 3 more. have a group of 4 and NOTHING else in the aquarium. I think a 10 gallon could handle 4 fish as small as those, as long as you have plenty of circulation to keep things oxegynated (i think i just made that word up). I dunno....some people may disagree, but i think you can handle 4 with nothign else. Look at it this way: if you were using, like, run-of-the-mill pinfish or something you caught down at the dock, you could put 5-6 in there and have nothign bad happen more than likely as long as you were on top of your game and kept thigns under control. I think people drop the numbers slightly as a precaution. Now, if firefish were 40 bucks each, ide say yeah 2 maximum, but you can get em for 10 bucks easy. What have you got to lose? Chances are everythign will be ok.
but i dunno.....let someone more experienced than me tell you a more precise number.