The fish you are considering are good tank mates for your clowns, but it would be asking a lot of your tank to add an additional fish. It's just too small.
By the time you add live rock, sand and anything else, that 20 gal tank is holding only about 15 to 17 gal of water. Typically you want to allow about one inch of fish per 5 gal of water in a SW system. So for a 20 gal tank, you can have about 4 inches of fish. Even small clowns are going to be almost 2 inches, so your at the recommended limit of the tank right now. Note that given time clowns can easily double that length. This would make things way over crowded.
I recommend that you stick with the fish you have, and not add another fish at this time. Start looking into a much larger tank, and have more fish in that.