This confused me for a while also....The not mixing of tangs is based on their family name. If you go to http://www.vividaquariums.com">www.vividaquariums.com</a> and then pull up the tang page, its listed with their common names and the scientific names. I believe there is something like 5 or 6 divisions of tangs. You shouldn't put two from the same, but can house two (or more) from different divisions.
Tangs not only get rather large, but are very active swimmers. So....you need to make sure you've got "open water" for them. Though....my big hoss that I just added (thanks BellyDancinBabyDoll!) loves zipping in and out of the tunnels and caves that I have. I've got a 235 with a Regal (aka Hippo), Naso and Sohal. I've played with the idea of adding a Purple Tang and a Kole.....but worried about space.