I do have a 3 gallon that is quite successful lol, but i did have my cubus setup as SW for a while. a Sunlit SW at that!(without a heater) It was able to keep mushrooms growing well. I took it down however because i am moving soon, and because my window didnt have the duration of sunlight that the tank ultimately needed (the tank got 3 hours a day of direct light, but needs more like 8 hours to grow into a nice full reef).
If the cubus got an actual lightbulb, it could keep soft corals very nicely.
I saved my cubus tank though, because its possible that i may set it up in the future with an actual light. My plan is to keep one maxi mini anemone and a pair of sexy shrimp in it when it gets revived
Also here is an example of something smaller than a gallon:
http://www.nano-reef.com/forums/index.php?showtopic=222438&st=0&start=0