I wouldnt do the turbo snail at all. ESPECIALLY in a pico. Those things knock over rocks that are 5 pounds apiece in my bigger tanks (which is one reason i never use them) but in a nano it would be total destruction. It would knock over rock and corals.
I'd go with like 3 cerith snails and maybe an astrea or 2. (astreas dont get that big. At least none of mine are and i've had em for a year).
I wouldnt get that blueleg either, because once you add those ceriths, it will be going after them to get their shells. Even if you put in extra shells for them, every hermit i've ever had has ended up killing a cerith for its shell.