An 8 could be ok for a betta, but you have no margin for error is such a small water volume. Most people who keep these fish in small tanks see them as disposable, and when they die in a month, they just go to Petsmart and buy another one for $3.99. They should easily live for a few years.
I personally wouldn't keep any fish in less than 20 gallons, but maybe that's just me. Maybe shrimp.