Coral is measured in many different ways in terms of pricing--actual length, area, number of polyps, color, etc. I wouldn't purchase anything if you don't have a reasonble idea of how big it is (most will list this somewhere), etc. Many of the soft corals and others would be fine. Mushrooms, most zoanthids, "clove" and "glove" polyps, star polyps, a good handful of leather corals (couple of exceptions). Once it is going strong and you get the hang of things, you may even be able to get away with some hardy stony corals, like bubble coral (aggressive).