Platy fish come in all sorts of colors, you could choose a more muted color or black Platy. They have milkyway Platy that's not too colorful. Most of all Platy shouldn't nip at your betta. It's not only about who your betta won't hurt, but about who won't hurt your betta :) My Platy hybrids and swordtails are my fav tropical fish. Swordtails get to be 3-4" though.
I had my Carnage in a 29g tank with a few types of disuse and they never bothered him. He was the last to go in though.