Agreed, prevention is better than the cure in this case in my opinion. Although I'd never put any average male guppies in with my betta, I've found him to live peacefully with endlers and venus guppys for years. He's also fine with my mountain minnows, black neon tetra, glow-lite tetra, normans lampeye and golden otos. Although sometimes I do see him eyeing up the lampeyes around feeding time. That said, I've introduced a fair few fish in there that were immediately removed as he clearly wasn't happy with them. (such as cardinal tetras which I thought was strange...)
I think what makes keeping a bettaso rewarding is the personality they frequently show. Each one is going to behave differently, and if you aren't careful about it, you have to be prepared for the possible consequences. =)