"All sea cucumbers are toxic" -- Julian Sprung, Reef Invertebrates
A sea cucumber was responsible for my first wipe-out. I had him for months with no problems, but during a tank move he got really upset and killed two sandsifting stars, 10 blue legged hermits, 20 astrea snails, a crocea clam, a key-hole angel...
...need I go on? Anyway, I'd go with a sand-sifter. They are amazingly efficient and very docile. You could add a couple of brittle stars for interest too.