Yellow tangs are one of the most aggressive and territorial tangs, and introducing another tang into the tank after the yellow has had a chance to claim dominance over the tank would likely be disastrous, as the yellow tang would likely kill the new introduction. As far as tank size goes, the yellow tang is fine in your 75. Many people say a 55 is fine, but most will agree that 75 is sufficient, although another large fish would likely overload your tank unless extra measures were taken. HTH