a shrimp goby, or possibly a basslet. Basically a small fish that won't attempt to live near the clowns host site. A pygmy hawk might work--depending on the inverts in the tank.
I wouldn't add an angel to a tank without live rock. A shrimp goby would be better--it could digout a home under the decoration. I'd likely consider adding some addition items to the tank--either live rock, lace rock, or more decorations--fish need cover.
You could get a Yellow Watchman Goby and a Pistol Shrimp. They form a relationship where they help keep one another safe and well fed. Here are mine...very cool to watch.
Most shrimp gobies will create this relationship with a Pistol Shrimp. I just like the Yellow Watchman the most.
I would recommend at least buying some cheap base rock...you can find it at some LFS's for around $1.99/lb and its often full of nooks and crannies. Most fish would appreciate that. Afterall you want to give them something to explore since they'll be in such a small environment (in comparison to the ocean) for the rest of their lives.
Oh, go pick out some inverts! I think they are the most fun to watch sometimes heh
I have a scooter with my percula's.... but get more rock or something else for cover... not enough cover could result in a fish death caused by stress.