I do agree, a 75 gallon isn't large enough for more than one tang. Hippo tangs get BIG and should not be kept in a 75 gallon long term. Unless you're planning to upgrade the tank in the future, you'd be better off skipping that option, IMO. A purple tang is going to beat the snot out of a yellow tang if it's just the two of them in a 75 gallon tank. You'd have to get VERY lucky to avoid that as a consequence. Blue reef chromis would give you the color you want, without the risk of outgrowing the tank or becoming too belligerent.
The fact that you currently have 2 tangs in a 55 gallon amy does not mean it's a good solution long term, IMO. People regularly push the envelope in the fish world and get by, but all it takes is one more small thing out of balance to lose the fish. As a hobbyist, our goal should be to provide the optimal environment for the fish, since we insist in cramming them in little glass boxes. Just surviving can cause stress factors that will later on contribute to HLLE and other health issues. Tangs are big, cruising grazers that are stunning to see in a tank with enough room in it. They eat constantly, all day long to keep up with their metabilism. That means they are also producing large volumes of waste, which is then diluted by a smaller volume of water. It's eventually going to catch up.
Just my $.02 worth.
Barbie