Some Tang cichlids are fine in smaller tanks. Many of the shelldwellers do quite fine. A breeding pair of Brevis can be in a 5.5gl actually, mostly becaue their territories are so small they rarely even travel far enough away from their shell to need a 10gl. For 10gl min, a pair of gold N. Occies or N. Multies are quite fine.
I would disagree. I have a 150 tang tank with
(4) Altolamprologus calvus Ink Fin
(4) Altolamprologus calvus White Chaitika
(3) Altolamprologus compressiceps Black Nangu
(4) Altolamprologus compressiceps Malasa Gold Head
(3) Altolamprologus compressiceps Red Lufubu
(5) Cyprichromis leptosoma Malasa
(4) Cyprichromis leptosoma Utinta
(6) Cyprichromis sp. Assorted Cyps
(4) Lamprologus brevis Katabe
(3) Lamprologus brevis Namansi “Sunspot”
(4) Lamprologus ocellatus Gold
(3) Syn Petricola
(2) Mustache cats
and my Brevis occupy a area that is about 25" square. They are constantly "on patrol" of their "base camp".