In a 120g, territoriality and fish compatibility aren't the same. My friend has a 120g, and he mixed a 8" yellow tang with a new Sailfin tang , a new Powder blue tang and a purple one. All those fishies would normally kill each other...no...they are friends. Weird huh ?
Also, 6 to 8 blue damsels are in it, a weird goby, purple pseudochromis, 2 maroon clowns and a butterfly. No problems, even tough it looks overcrowded.
As long as they have a lot of rock structure and places to hide, it can be done. This guy is lucky and he gambled, but hey it works!