Don't get a tadpole unless you are prepared to keep the adult. Anyhow, the most commonly found tadpoles at fish stores are American bullfrogs, which get too large for a five gallon.
If you get a newt, stick to a Cynops or Notophthalmus species, and only one or two individuals. Other newt species grow too large. I know you are looking for herps, but frankly a five gallon tank is much more suitable for shrimp or dwarf crayfish than for any herp.