One or two of the little African Dwarf Frogs will go just fine with your betta. They are fine little pets and I'm sure you will enjoy them.
Just make sure that the tank is well covered, with no little spaces that they might jump through. They also appreciate some kind of little cave to hide in sometimes.
If you haven't seen this link, be sure to check it out to be sure you are not getting African Clawed Frogs by mistake.
Thanks. I've actually had ADFs before. I love them. I was just concerned that it was not enough space for them. My female betta is lonely in her 2.5 gallon tank...