That depends on who you ask. I would not call a nano, 'nano' if the actual system contained over 40 gallons of water. My 20g nano is a single 20 tank with a bakpak skimmer and no other water container. Many of the nano guys would say that a 20g is not 'nano'. This has been argued on www.nanoreefs.com more than once.

If you want a more successful small tank you should definitely attach it to a sump, refugium, or other tank. My 58/40 system has become much more stable since I added the 40g tank to the already running system. I have my hard corals in the 58 on top which has high light and many of my softies, mushrooms, and zoos in the 40 on the bottom.