The serpae tetra mentioned is a bad bet with the betta. I have 3, but I refer to them as the "raptors of the tank." They're very fast, very active, semi-aggressive, and at least as far as I've seen, they always out smarted the other fish when it comes to getting food. My serpaes, when well fed, leave my other fish alone and I like them a lot but . . .
They love to nip at each other (but their fins and body type are meant for this behavior), and if not kept in a big enough school, have a tendancy to rampage the rest of your fish.
A slow moving, long finned betta with serpaes would be little better than throwing an angel in with a school of tiger barbs . . . The other blood tetras will act just as bad as the serpaes. There are more docile tetras . . . do your homework before picking a species.