you can venture somewhat sideways of that standard ratio though as far as some species of marine betta or harlequin tusk are not notable as being great and fast swimmers and on the other hand tangs need far more room especially length to be comfortable in their surroundings. You could have a large angel in a smaller tank generally than a large tang just due to the fact that a tang has sudden bursts of speed especially the sailfin tangs in which they need longer tanks than the ratio provides them. You just need to make sure and do research before you purchase anything from the store like how large they get, what they eat, how much rock they like, where did they come from things like that. For the most part unless your completely loaded no consumer goes to a used car lot and says I want this one because its pretty and then takes it home and finds out it cant care for it (oil changes too much, gas too high, maintenance far more advanced) I am always around if anyone ever needs help in deciding what fish or corals will properly coexist inside of their tanks, . . . . and off my soapbox lol
let me know though if anyone needs help on aim is the fastest way of course