It is very important to know what type of starfish you are thinking about, and what type of crabs you are talking about.
Keep in mind that all seastars need mature tanks, hopefully at least 6 months old. IMO, your tank is too small for most of the reef safe stars, for example, which only feed on LR, are very delicate, and can not be spot fed. Most will starve to death in a tank of your size. Non reef safe stars like chocolate chips are tougher and can be spot fed, but may eat things like corals, snails, etc.