The Ctenochaetus</em> tangs stay on the smaller size for tangs and could be housed comfortably in a 250. The Zebrasoma</em> tangs could fit as could a Paracanthurus hepatus </em>(if the tank is 6'). All the Naso</em> species need 8' tanks or longer.
I would recommend only one tang per genus to reduce aggressive behavior.
I have a 300 and have the following tangs: Purple, Powder Blue, Blond Naso, Chevron, Clown, Achilles, African Regal, Desjardini Sailfin and a Tomini. I know, CRAZY!!