As Jouney said, you can mix tangs in a tank, but you need the following:
- Large tank with plenty of space for the fish to duck away and graze.
- Mix multiple of the same type (i.e. 2 hippo tangs, 2+ yellow tangs, etc.)
- Mix different genus and unsimilar body type.
Tangs can be nasty fish because of their blades on their anal area. Fish they do not get along with tend to receive nasty slashes and worse death.
IMO - with your 75, you can keep your yellow tang, but would never add another tang as it will eventually cause space issues. If you had a 180+, the mixing a few tangs would be ok.