One cucumber would be OK - as long as it can not be mushed by any return pump, etc (they are VERY toxic, and will kill your entire tank if seriously stressed). They prefer to live on rocks, though, and not many species will live on the sand. If you manage to get the right "turd" type, though, they do a great job of eating diatoms.
I would NOT place any "hunters" in the tank, though - many starfish and other so-called sand stirrers actually "stir" the sand whilst hunting the smaller critters living in the sand, and that is not good.
Hennie