Being different genus', zebra and acan, you may be ok; although there are several good points in counter. A 90 may be too small for two fish of this size and would leave little room for escape, should one turn on the other. Purple tangs are considered by many to be the most aggressive of their line. Going in after the pb, it may counter that but you should have a fallback plan in place, should things not work out. A good trick for aggressive tangs, in particular, is to hang a small mirror on the (out)side of the glass. They're usually more aggressive toward their own kind and will often fixate on their reflection, and forget about the others in the tank