Everyone recommends them in groups of 12 or more to prevent them from picking too hard on each other. I don't think you want 12 2-6" fish in your 15 gallon tank. Cichlids are fun and easy, maybe they would be best for you.
bigspizz is right ive seen them at 5 inches in a 20 long,my friend boughtt 6 of them when 2inches then the largest one killed off the others now the 5in one lives alone.