Unfortunately, that betta will cause problems( a it not because the betta is aggressive with other fish, they are very good with most others) . If you have a bigger tank, like a 10 gallons, you could mix many fish, but since its a small tank, your options are limited. You could add\
2-3 cory catfish
or
2-3 platies( maybe)
These are basically what will work in your tank. I would watch the platies, though, because they can look like bettas to bettas, but this is more common with male guppies.
Hope this helps!!!