5 tetras is a good idea. I had 2 guppies with 5 tetras and it was pretty cool. I would just watch your water parameters because when i had them in my 10 gallon I would do about a 50% water change a week because the nitrates went up pretty quick. I also had a bamboo shrimp and 3 snails with them so that added to the problem.
But that stuff aside, a school of tetras is pretty neat.