You must have a DSB to maintain Nassarius, this is a must as they are sand creatures basically. If you have a DSB I HIGHLY recommend these guys.
Margarita are almost worthless. Ans STAY AWAY from the "cute" little Bumble Bee snails! They are indeed predators.
Get a couple of Fighting Conchs (they stay small), or if your tank is large enough, get 1 Queen Conch(she can get quite large.)
They are eating machines. Quite cool, and fun to look at and watch.
My .02
HTH
Hermit