Personally, I believe the size of the tank really isn't an issue. I mean, you couldn't have a 4 inch tang in such a tank, but, you could have a nicely stocked 2.5 if you are smart about your choices.
But the real issue is supporting the life in the tank. And the demands vary based on what you put in the tank.
First, you can't over crowd the tank, but, I can see having a Nem and a clown with a few coral pieces and modest clean up crew.
I can't imagine having a skimmer of any kind on such a small tank, hang on back or otherwise. The key to success here would be water changes. I would do 1 gallon daily.