depends on what you like and what you would like them to do. If your sand and live rock have matured then usually 6-12 months would be okay for most stars. Brittle stars are scavengers, sometimes get big and sometimes will go after fish in your tank, especially if they are sick or weak. Sand sifting starfish are very docile, hang out in the sand, turn the sand bed over and eat pods in the sand. You usually can't have a whole bunch of them unless you have a large tank with lots of surface area of sand, and they spend a lot of time invisible in the sand. Linkias are beautiful. They come in bright blue, orange, maroon and other colors, but they are sometimes a bit tougher to keep. Spiny stars come in cool colors as well, but often eat corals.
Not exactly all inclusive, but at least should give you some idea.
Mat