allaboutfish
Aquarium Advice Addict
haha... you could just see if they'd take him back if you needed too.
In my humble, party pooping opinion, its really not nice to add more fish of any kind to an uncycled tank.
I have three 5 gallon tanks with only bettas in them and the bioload is plenty. But as you asked....once the cycle is complete, one or Endlers might work if you can find them.
A killifish of a shortfin, small variety would work too. There are numerous varieties that stay under an inch. Or there are several types of nice looking nano rasboras on the market these days. They stay small and could probably handle an aggressive betta.