Definatley watch your water parameters.
Also, with the Biocubes, make sure to keep the filters clean. I recommend cleaning them once a week religiously!
Smaller tanks can be difficult, especially in the beginning, but a pair fo False percs are good fish to start with, they are hearty and can tolerate some change in the water.
Watch the fish closely for any disease - drastic water paramter changes can stress these little guys out quickly and they could end up with saltwater ick.
Make sure you tank has cycled before you put the fish in. you might consider putting in a cheap dansel or chromi as a "test" fish. They can help the tank complete it's cycle.
The rule of thumb for saltwater fish is 1 inch per 10 gallons, so you shouldn't have more thant 2.5 inches of "swimmers" in the tank. non-swimmers like crabs, shrimp, snails etc.. don't count towards this though.
Gobys can be a lot of fun, we have a pistol shrimp and watchman goby, and the kids LOVE to watch them do their thing!
Make sure to do regular water changes, 10% a week is good. (2 gallons a week would suffice for you after cycle) - make sure to vaccuum the sand a little when you do the water change.
If you get algae, don't panic.... it's going to happen.. it always does. This is usually a sign of high nitrates. more frequent water changes (though no more than 25% at a time) can help "starve" the algae out. There are prophelactic(sp) methods to get rid of them, but the fewer chemicals you have to use, the happier your swimmers will be.
Make sure to quarantine any new fish. If you can't do it, ask your LFS to do it for you. Saltwater fish are more prone to disease, and once they are sick, they can be hard to nurse back to health.
Anemonies are GREAT for small tanks as well once you have the tank stabilized. You might even find your clowns hosting someday!
Good luck with the biocube. There are many many people out there that have had success with these little tanks in a saltwater setup. I have two of them successfully running.
I'm a new member, but I have lots of experience.