People may disagree, but I find the thought of just a single fish (tang, for example) being adequate for a 75g tank alone and nothing more absolute nonsense. If we go by things like that, that's three tangs for a 225g tank which would be downright ridiculous.
I have a pond which I have Koi in which supposedly require 1000g per Koi according to some 'experts' because of the size they can potentially grow up to, but the fact of the matter is, no one in there right mind gets a singular Koi for a 1000g pond, and in aquariums, it's the same. Sure, there is a limit, but I think there's a lot of ridiculously drastic people out there who have the thought that these fish really need that's sort of space on their own. If it was really the case, no one would be in this hobby because no one can afford to get a tank covering their entire walls with 5 tangs alone. It's just not a viable hobby if we're going to be absolutely correct with the specifications of each fish. Even on LiveAquaria, a Koran Angelfish is written down for a minimum tank size of 250g! Yes, a fish that would become no more than 3" according to them needs a tank of that size. I don't know anyone who has a single angelfish in a 250g tank.
That 'basic rule' in my opinion, has always been absolute rubbish. You should always consider the space they need, but you definitely have the ability to have more live stock than what a lot of people would say.
Definitely another tang, a couple of clowns if you wanted and then see from there whether your fish and how your tank handles the capacity, which I doubt would be a problem anyway.
Sorry about the above, I just felt like this was a chance to get out my thoughts on these 'basic rules'.