First patience. Second thing is research and cross reference anything a LFS
tells you with a forum on the internet. Third is get a test kit and learn to use it properly anything you add to your tank make sure you have a test kit to check it.
A small tank is a lot more work than a larger tank. I have a 2.5, 8 and a 45 gallon I spend more time on the smaller tanks than the larger once including testing, water changes, top offs and even research. Jnam has a pico on this site. You wanted to know about what you could stock a pico with here is a link to help you get started on research lgreen's Ultimate Guide To Nano Fish - Nano-Reef.com Forums
Aparting thought don't' cycle with fish and any fish you put in make you you want it in there they are very difficult usually to catch without having to take the whole tank apart. Damsels is one of the first fish people start with and find they are a mistake.
You are in for an adventure.