If you have had these tangs in a 60 for a few years, they are probably pretty stunted by now, and you don't have that long with them, anyway. Since you know better, and don't believe anyone here...prove it to yourself like this...
Go to an LFS and take a pic of a wild collected fish about the size of yours...and look at its eyes in comparison to the fish in your tank. (Or get a pic of a wild fish that looks the size of yours online) The eyes of your fish will be much bigger. This is because the internal organs still grow, while the outside does not. Basically...your fish will soon have their internal organs rupture, and die.
The shark is fine in the 60 as a baby, yes...but they grow fast, you should have another tank already. And yeah, your tangs get along with each other still in this small space...but they will probably pick that newb that looks like he probably eats too much to death...or else they won't, and it will be a race to see who dies from the small tank first...I am betting the tangs, shark has more time...