As far as stocking goes, there is a general rule of thumb of 1" of adult size fish per gallon of water. This pertains to fish who's full grown size is under about 3". So for the type of fish you are interested in you would want no more that 30 inches.
Another thing to consider is what region of the tank the fish inhabit (i.e. top, middle or bottom). You can get away with a little overstocking if the fish inhabit different regions and you are willing to do more waterchanges. Remember, the more fish you have, the more
ammonia that they create, which ultimately leads to more nitrates.