I have a 4g pico and it's pretty much full of corals and inverts. Porcelain crabs, sexy shrimp,pom pom crabs, peppermint shrimp(small ones),snails and hermits would be fine to add as well.
The 13 watt bulb over a 1.5 should be enough for zooas and mushrooms. I've been paying about $10 a shroom at my
LFS and $15 or so for small zooa frags with maybe 10 polyps.
Make sure you post some pictures whenyou stock! We love photos!
edit** almost forgot, just do water changes you dont need a skimmer