OK, a 20 is too small to keep an anemone in. They require pristine water parameters and that is tough, if not impossible, to do in any tank under 30 gallons. You also need a lot of intense lighting and in a small tank like that, it will cause the water temp to be high.
No, you don't need an anemone to breed clowns. Most are bred without one. Get the book Clownfishes by Joyce Wilkerson, she explains step-by-step how to breed clowns.