Dwarf angels, basslets, hawkfish, gobies, clowns--there are a variety of fish that will be happy in a 55. This doesn't mean you can get lots of them, because the carrying capacity of SW tanks is way, way lower than that of a FW tank. Which means, large, heavy bodied, waste producing predators will not thrive in a small tank due to water quality issues. Compound poor water quality with a small tank that induces stress (and yes, a 10 inch fish will be stressed in a tank that's 12 inches wide), and you end up with stunted, disease prone fish with shortened life spans. Make that a trigger, and eventually, it will likely snap and kill every other fish in the tank.
If you're waiting for someone to tell you that going under the minimum listed size for a SW fish will work in the long run, it's unlikely to happen.