Just the betta would be absolutely safe and fine. Some of the smaller one-inch fish fish, like a glowlight tetra, would be okay in a small group, but really, the only thing I'd keep in a 21 liter tank is a betta.
And they're so cool! They all have their own personalities, and they flare to get big, and you can teach them to come to you when you approach them, and they wag their tails and look so happy to see you.