What fish do you have already. If you want something small and entertaining a cpo is very good at eating snails. Not a fish, but as long as you don't have anything with long fins should fit in fine.
I'd love to have a cpo, but I intend to have pygmy corys and a show fish, possibly a dwarf gourami.
Don't CPO's burrow and uproot things? I thought most crays did.
I considered getting an assassin snail or two, but they burrow.
Gouramis are a good suggestion. I've never found a dwarf cichlid that seemed like a good candidate for living singly in a 20 gallon tank.