When buying fish always try and buy fish with different dispositions, shapes, and colors. For example. Blue and yellow tail damsels swim through the rock work and can be paired with green chromis whom swim at the top of the tank whom can be put with a goby who stays at the bottom of the tank all of whom look very different from eachother..... Just a long example.
I'd suggest not buying a fish unless you have the tank for it already. Never assume you will upgrade till you actually do. Most people do this and a rare few follow through and get stuck dealing with large fish.
I would look for a small, fish handbook by michael scott called marine fishes which details most any fish you'll find in the stores. Follow his guidelines and you won't have problems.