Assuming that the 6.5" central ceiling height (Duck!) is actually 6.5' (Duck, if you play in the NBA ;-} ), that's your overriding concern, right there. I'd probably try to find something with a tweeter that had limited vertical dispersion like some ribbons, planars, or possibly the Gallo tweeter. Beyond that, the room (again, assuming that 16X21 represents feet) should allow most speakers to work pretty well.
Good Luck,
Marty
Good Luck,
Marty