Actually, the inch per gallon rule is a total myth and can greatly endanger fishes' lives.
I've been to several cichlid meetings with cichlid experts speaking in the past couple of years and they've stated that is a good approximation. I can tell you that we have a total of 14 fish in our 125 gal and we will be stopping there as they are all growing and we will have to go to a 265 sometime this year, probably.