THe blue leg hermit are a little more agreesive then the scarlet leg hermits. The blue legs will kill the astrea to the shells, but so will the scarlet hermits. But the scarlet are less aggresive in that prespective. You could always throw in some larger empty shells in. As for how many of each I couldn't tell you but on this site they sell reef cleaner packages which should give you a dea of how much to get.
Anyone Correct me if I'm wrong.