IMO I dont think your tank is near ready for a starfish of any sort. The reds or blues are difficult to keep. As for clams they need pretty good water parameters and decent lighting. I dont know what you have for equipment so I cant really comment. It's best to start researching all your livestock purchases before getting anything.
www.liveaquaria.com has a good database of info to a certain point. Check it out. But some of the info is not always right but it's a good start to ryour research.