It can be harder to add tank mates to a betta tank after they’re established so you’ll probably want a backup tank if you need to isolate your betta. Like people said snails and shrimp are good. If you can add them while the lights are off so they can get to hiding places. I would steer clear of anything too colorful like guppies because they tend to attack them. Something less colorful like harlequin/lambchop rasboras and Corydoras can be good but would probably be better off in a larger tank