Smaller tetras, I'll argue, can go in a species only 5 gallon tank.
Tiny neons, lemon tetras ect..
People may chastise me for this, but they certainly can thrive 6 fish at a time in a 5 gal. The dimensions are so minutely different from a ten gallon other than height, it doesn't matter.
Take or leave my advise, but it is what I will always believe.