I have 2 shrimp and I typically feed 1 choc chip starfish per month to them. They are in my sump.
They will munch on the starfish for a couple of weeks, then i pull the remains and let them go for another couple of weeks without. That has been my routine for the last 2 years.
A whole starfish is best IMHO. The legs don't last long and regardless of regenerative properties of the starfish, they do not grow back fast enough to keep harvesting legs enough to feed. Just my experience...I might just be impatient!