A lot of people don't like Bristle worms but in my opinion they are the most valuable clean up crew available. A piece of fresh live rock will almost assure you of acquiring some worms. If not I would imagine there are many hobbiests that would be willing to give some away.
The 20 might be a bit smallish for a Cucumber but, one advantage of a Cucumber is that they can grow and even shrink to match the food supply. Very few other animals can survive shrinking down to match their environment.