I think you're in going way too fast brother.
You put in too much too fast. (fish, star, anemone). I highly doubt the anemome would have survived even if the starfish didn't eat him.
Putting that starfish and the anemone in a freshly started tank is a bad idea in my opinion and that guy at the pet shop should be smacked in his **** head for selling you that live stock. Not to mention that fool sells you all that without informing you that ammonia, nitrate and nitrite kits are a necessity for a new tank
Bring the star back and get more live rock if you can...at least the chromis have a chance to survive so give that a shot.
Good luck and find a new lfs
if you can.