None.... balas get around a foot long if not more and like a foot wide. I wouldn't even stick any in a 10 gallon tank just to grow out, the space is to small and could prevent growth and health problems in the long run.
I would think 55 is small in the long run so about 100ish+?
I know that for a group of 4 (and they really should be in at least a group of 4 as they're schooling fish) you need a 125g (6 ft long). They can get over a foot long and swim fast, which is why the tank needs to be so large.
I agree that even a 55g is small. I currently have two 7" Balas in my 55. From past experience, I can tell you that a Bala that size will spook very easily when it is confined. On several occasions, my Balas have hit the glass lid hard enough to lift it up and cause it to crash into my PC light fixture which is a good 4-5 inches above the tops. Fortunately, they suffered only injury to their pride.
The rest of the species in my tank will just about come through the glass to you when you approach the tank (conditionally trained), but the Balas take off like a Tomahawk missle a huge majority of the time. Currently trying to find them a larger habitat.
As everyone else has stated - you cannot keep a bala in a 10 gallon tank. Mine was in a 55 and was rehomed to me since I had larger tanks. It is in my 6' 100 gallon tank and he could use more room, however he has grown nicely in this tank.
There really is no such thing as a freshwater "shark". They are actually a type of catfish and as such do get quite large. The iridescent shark gets even larger yet!
As stated...not a good tank size for them. The only way you could use a 10g for Balas is if you were using it as a quarantine tank for a few weeks to a month at most. A group of them wouldn't last long in there especially with the bioload.
Red tailed shark? Red tailed catfish? Shark will get around 4 inches though I have seen 6 inches and still wouldn't keep one on there. Red tailed catfish get HUGE... like super duper huge and you won't be able to house it properly unless you got like 1000 of gallons :S.
Could always start a thread asking what is safe to house in your 10 gallon tank, such as what species to make the search easier on you.