depending on the size of tank you have, you can mix some species with others. Yellow tang and Hippo tang should probably be ok together. When mixing tangs, get tangs of differant shapes, and introduce them at the same time, both should be about the same size.
Most important though is that your aquarium is of proper size. A mature yellow and Hippo should be in atleast a 90 gallon aquarium, the bigger the better though. And please dont overcrowd, tangs can develope ick very easilly when they are stressed.
gl
Sea Goblin