This is a sometimes debated question.... My generic response is the betta should be alone. I have seen bettas living ok with other fish around the same size but it isnt something that I would recommend... If your going to try it, maybe a large gourami? I think you would definately have a problem with any smaller fish like most tetras.
One thing I recommend to people who like bettas but want a bigger tank is to put a tank divider in the middle and get one betta for each side. If you get a clear screen, they will see each other and flare up.
HTH