Sam,
A lot of it is trial and error. Each betta is different and has a different personality. The ones you absolutely CAN put in with a betta are:
Cory cats
Otos
Mystery Snails
Those three kinds of tank mates will not hurt your betta. Your betta may pick on them, but they will not hurt him.
Anything else may pick on your betta. Or not. I wish we could all give you a definitive answer, but other than those three -- which I think everyone can agree on -- anything else is hit or miss.
Roan
As an aside: has anyone else other than me tried a pleco? Which is the smallest sized one? I had a common temporarily housed with the betta and it worked out great.