I have to disagree with Siva on the Tetras. My Tetras have only been stressed and territorial in full schools. Just ended in disaster this last weekend when I got black skirts to finish the school for my white skirts.
Nevertheless, I too don't recommend having any other fish in there... Bettas aren't friendly for one, but two, the five gallon tank isn't really going to be big enough. Even in a ten gallon, the relatively low bio-load is made up for in the fact that a Betta is very territorial and requires a lot of space.
I do agree that shrimp might be good tank mates... May get eaten, but a ghost shrimp is less than a dollar, usually 30 cents where I've seen them, and good cleaners too.