I'm trying not to sound offensive, but please pause and think about what your so-called "experts" are doing, and use some common sense. You're probably aware that a desjardini salfin tang would grow to at least a foot in captivity. Does it make sense that it be in a 90 gallon? It would barely have room to turn around. You'd probably argue that your "expert's" salfin never grew to that size, and I'd bet it wouldn't, because it's severely stunted. A stunted animal typically has two kinds of behaviors: seclusive or aggressive. A baby salfin tang would surely get above 10" in 5 years with proper care and environment. If it doesn't, then something is wrong.
My point is that even though I see where you are coming from, I do not agree that the approach is appropriate. Keeping tangs (or other similarly large fish) in a smaller-than-recommended tank will not necessarily guarantee aggression or disaster, but it's simply not a good environment or welfare for the fish. I know people who've kept a hippo tang in a 55g for over 10 years. It never grew past 4". Is it surviving? Yes. But is the fish happy? I bet not. Being an aquarist, we should constantly have fish's welfare in mind instead of our own pure greed or desire to keep whatever we like in a small tank. People have different levels of "success," but for me, success is measured as the overall happiness and health of the fish, not whether the fish is surviving.