Just put one cocktail shrimp in there. As it decays it will cycle the tank.
Even if you don't add any live rock, you will need some kind of decor so the fish feel secure and have places to hide, sleep, or graze off of.
Damsels can be very colorful and very interesting, but they can also be very, very mean! Be careful what you choose and make sure that if you do decide on damsels that they are the last ones in!!!
Good fish for a 30g: firefish gobies, small clownfish like perculas, or oscellaris (sp?), yellow-headed jawfish (these can be awesome but they need a deep sandbed to have their burrows in - like 6"), royal grammas, as far as damsels, maybe the yellow-tailed damsel or any of the chromis.......whatever you decide to get, just remember not to overstock the tank! And don't add the fish too quickly either. Maybe one fish every week or two.
Good luck!