the most important thing's are, what are your tank conditions(size, age, lighting, filtration, water parameters, other inhabitants(fish, crabs, shrimp)) these things should be considered first- will they support a clam( what type of clam).
then the same thing for the starfish.
clams wont eat starfish but some stars will eat clams, corals. so you need to look into the starfish you might want and see what there needs are, and then we can go from there.