we at the BigAl's in newmartket have a fully mature purple tang in a 30 biocube. Happy as ever, swims 24/7. has once since i worked there gotten ich, which was treated and gone after 2 days. in the 75G we have a fully mature yellow tang and a fully mature flame angel. Never had any problems.
Remember, whatever you read up on in both books or online, those are just guidelines as what to do. The important thing on keeping tangs in anything less than 100G is that your water parameters are in check and if less than 100G do only 1 tang, and no other large fish.