If you add them at the same time, I'd do a pair of cherubs and a potters. With enough hiding places, I thinkit would be OK.
For a tang I'd look at a tomini some times called a flame fin, love these guys. Keep in mind, a 36" cube is 48" on diagonal and depending upon you rockwork may have more free swim space then many 6' tanks.
Pair of Orchid dottybacks, if you like pairs, you'll love a pair of ODs, just be patient as they are slow to become bonded but once they are the interaction is really cool. And they take up zero swimming room as they zig zag thru the rock work.
And, maybe a trio of anthias depending upon the rockwork. And see how it goes from there....
.... Hmmmmm that sounds sweet, I think I need a cube tank now! Good Luck!