I would agree with Sugarfox03 on this. I have 3 in a 24g aquapod, with 1 neon goby, 1 juvenile pygmy angel, and 1 disappearing wrasse. Even with a skimmer (the sapphire skimmer in rear) pulling out plenty of brown skimmate, and 10-20% water changes a week, I could not see how I could handle an additional load without messing up my water, as well as having too little room for the fish to call home. While I'm not lazy, I do think that a small tank really calls for light stocking (some might think I'm overstocked) for the health of the tank inhabitants, and for my own sanity.
I also don't keep SPS, because if I did, I would feel very leery about any fish to feed in the same small tank. I know that may be very conservative, but I'd hate for something to die just because I pushed my own limits. Folks here with far more experience can be successful at keeping such a tank healthy, but I don't think my personal ability is there yet. Absinthe and colinadam are both right on the points of careful attention to maintenance, and also to fish compatibility, which is a real issue in so little water.