My 5.5g is packed, and I even made a little skimmer for it. I have a few sexy shrimp in there, a couple boxing crabs, four porcelain crabs, a frilly-arrow crab, and a few hermits as well as the goby.
The secret is that its not really a full-on reef. I only grow hardy mushrooms in there (its nothing but cobalt blues, bright oranges and watermellon reds with some metallic green shrooms... quite the eye-popper for a shroom only tank). I suppose, I do have some pipe-organ in there as well as some xenia... and a frag of monti-cap, but for the most part, its nutrient absorbing soft corals.
AND, the main reason I can have such a high load inside of it is because I change out almost all of the water on a weekly basis with the main systems. I remove about 4 gallons, then take 4 gallons from the main reef, and put that in the nano. Then I replace the 4g in the main system. It makes for a small percentage change every week... very effective. It also keeps the nano in tip-top shape without any major filtration but the built in sump and skimmer.
If you have a larger system to 'borrow' water from on a regular basis, your odds of success are greatly improved.