What corals you can have will be limited by the light. Most picos do best with soft corals. For livestock you can do sexy shrimp or pompom crabs. With a pico you will have to do top offs several times a day unless it is enclosed. Enclosed tanks have a special set of problems so it is usually best to go with an ATO for top offs. The more stable you can keep it the better. For a first tank a pico can be done but it isn't easy, very little room for error.