IMO it totally depends on the fish. I mean porker puffers averagely reach 11" in aquariums (some get bigger, some get not so big) but you couldn't pay me enough to put one in a 55gal tank. They produce too large a bioload and are too active. Again, with tangs (although there is a lot of dissention on this topic) the smaller tangs get from 7-9" long, but they're streamlined swimmers and very active...so I wouldn't keep one in anything under 75gal. I know there are people who will tell you that they have three to five tangs in a 55gal. What they rarely tell you though is that they're either upgrading the tank when the fish get larger than 3" each or they forget to mention that they've lost ten tangs or so because of this.
The only real way to determine how many fish you can have in your particular aquarium is to study the fish before you get them and study the fish you plan on getting, watch the cycles (watch for mini-cycles) and see how your tank is doing overall. I've known a lot of people who push the guidelines because they've wanted an active tank. And I can't tell you how many times I've had to hear them freaking out because the tank crashed, or one fish snapped and started killing all the other fish, (most likely to do this: Yellow Tangs.) especially new additions.
