Can stern box launchers coexist with a forward six-inch gun?

Is it necessarily unsound to put two box missile launchers at the stern behind a forward six-inch gun?

Not necessarily; if the box launchers fire to port and starboard and the gun sits forward of them, their arcs need not interfere.

The answer depends on how the launchers are oriented. Stern launchers directed outboard can remain clear of a gun mounted ahead of them. Weapon integration depends on geometry and firing arcs, not simply on which system appears first in a side profile.