I thought the "general rule" for salt water was 1" of fish per 10 gallons of water?
even with your 5 gallon refugium won't your actual water volume be under or just about 10 gallons?
I had thought it was impossiable to do but I found a few sites that listed small saltwater tanks down to 4 gallons
[url=http://saltaquarium.about.com/cs/msub10s5to25/]small salt tanks - maybe helpful info?[/url]
It'll be interesting how it goes once you add fish.
Good Luck