It sounds like this is your first saltwater tank and if it is then a 6 gallon is kinda small to start with as a 20-30 gallon tank is recommended. If you do decide to start with a tank of this size I would have to agree with the above post about keeping the tank invert only. Keeping the tank invert only with put very little strain on your system as far as bioload is concern but would still provide you with a colorful and active tank. A cool centerpiece invert such as a Cleaner shrimp, Fire shrimp, Coral Banded shrimp, or even a trio of Sexy shrimp would be nice.
If you did want to add a fish I would only add a single small fish such as a Goby. Species of Goby that will work include Trimma, Neon, Clown, Masked, Catalina(prefers cooler water), etc.