Tank size/Fish compatibility

[asylumdown]

I'm in the early stages of planning a new tank, and I wanted to see what you guys thought of these fish in the dimensions I'm planning-

Tank will be 36x36x22, so 123 total gallons (though I'll likely have 2 inches of sand).

I don't want many fish, but the two that I really hope can live in a tank this size are a powder blue tang and a copper band butterfly.

what does everyone think about those fish in a tank that size? I know people normally say 6 foot minimum for a tang like that, but I have zero experience with cubes, so I'm not sure if that gives you more options in terms of fish.

Any thoughts?