My sea cucumber even cleans the aquarium glass. When it does that, I'm reminded why I named it 'Mr T'.
It's actively cleaning the sand bottom even the area of live rocks that doesn't have anything. I wouldn't suggest you getting a big cucumber as it has tendencies on 'throwing it's weight around' in the tank. Having one should be enough.
From what I heard, it release toxic material when it dies. So if you're decided to put a sea cucumber l suggest you keep a close eye(looking intensensely, not closed eye LOL).
Oh, just to share my experience, when I placed mine in the tank, it didn't move for a few days, not an inch. (at that time, I didn't know they do the toxic-death thing) But I let it be and all of a sudden it moved and start getting to work. It's active ever since.
Also, if you are looking for critters to move your sand, I suggest you also consider snails, not turbo snails, a different kind. But sorry, I can't help you with the names.