Im using 4 tangs in a 6ftx2ftx18in. So I have to have 6ft because they all need their cave to sleep in.
Some tangs are for sure more aggressive swimmers than others. yellow eye kole tang and powder blue tang seem to need bigger tanks to swim in.
yellow tangs, sailfin, hippo, vlamingi need their own caves and they still use the 6ftbut are not as aggressive swimmers.