Hey! I just got the following type of fish:
There were a few things that I wasn't expecting, though. When I bought him, he didn't have the metallic colors. Also, he has almost as much spunk as my betta (right now he's swimming up the current of my filter and letting it push him down, rinse and repeat). They eat the same food, and he and my betta get along awesomely! I wasn't sure, since he *is* a little bit larger than my betta, but they seem to be buddies. I just wish I could remember the name of his breed!!
You could also try neon tetras:
They're a lot less hardy than bettas, though. I was advised to buy them in a school. I've also read a few things on here about them being finnippers unless in a school. School size seems to vary from person to person, but I'd say anywhere from 4-6 is reasonable. Mine were a bit stupid, though. They committed little fishy suicide by forcing themselves into plant areas that they couldn't get out of. I woke up during the night and found them dead in weird angles in the plants. Poor little guys. Speaking of little, they are rather teenie.
Anyways, I hope this helps! Keep in mind that not all bettas like tank-mates, but from the sounds of it, yours doesn't seem to mind. (: