Personally I think it all has to do with swimming space. For instance a 75 gal
tank with little rock work can Have more open swim space then a 125packed with rock. Yes you can keep a yellow tang in a 75, yes it will live but at some point it will outgrow the tank. Ie a 9" fish is way to big for a 75gal tank. This is why they state putting them in a 125 to compensate for the larger fish. I would say yes just as long as you do plan to upgrade in the near future. And as a side note for tangs to fully grow and mature they need a 300gal+ tank. In mr.x build thread this is one of the reasons he is replacing his tank in the wall for a larger one. I think the Dimensions were like 8ft l x 4ft w x 2ft h.