Tangs dont need to be in groups at all, in fact you'd probably have better luck if you get different body shaped ones as they tend to be overly aggressive with tangs that look like themselves.
Tangs and harlequins are fine together, the tusk is a semi aggressive fish in my opinion as are most tangs so that'd be a good match. AS the tusk is a wrasse i would suggest a screened top or lids just in case. hate to see $100-$200 jump out of a tank on ya. lol
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.