personally i would keep about 12. if the tank is 7 feet long and 2 high thats a pretty good size man. the only thing better would be 2 wide and 18 high. the number of fish isnt the really crutial part its the water quality. if your filters can handle the waste produced by only 3 p's then thats all you should have.......... if it can handle 12-15 then get 12 to 15 and thin out the group when the time comes. never start with small numbers because as the fish grow and you might want to add more then you have to find fish around the same sizae as your current ones.......... much easier to thin them out later if you have too many big guys.
id start with 12-15 juvi or 25 -30 dime size. or 12 adults