Does the Maroon have an anemone? If so, I'd recommend something small and fast for a tankmate.
Seriously.
Aside from territorial issues, the Maroon will grow enough to become your "one large fish." It will likely be dominant, esp being in there first. A Tang in that size tank - some people do it, but after seeing a couple of Tangs pace in my own 6-ft tank, I won't ever do it.
The biggest thing I would put in there now would be maybe a dwarf Angelfish.
YMMV, IME, etc...
