It's called "Hyper-Stocking" (my term, not an official one). The basic principle is that you cram so many fish into a tank, that no one fish can establish a territory, and any aggressive is spread out between all the fish. There several notable problems, the first and foremost, is bioload. 100 tangs in a 600gal is one tang for every 6 gallons of water. That would require massive filtration, and almost hourly water changes to maintain water parameters. The secondary issue for hyper-stocking to work, is that you actually must hyper stock. You can't just have 4 or 5 tangs in a 135gal and expect it to work. The cost for the hobbyist to do this is huge. I really don't think you have $4,000 for a 100 yellow tangs, huh?
There are of course, certainly moral objections. The fish are at constant stress levels. There is absolutely no where to go if a fish "wants to get away" and there's no caves for the fish to sleep in, etc. That is no way, shape, or form, how a fish should be kept. Most of these tanks are kept at constant hypo or with constant copper in the water. When leads to prolonged health issues.
My advise would be to enjoy the Sailfin and Hippo you have now for the time being. They both will eventually outgrow that 135gal.