you need a ten gallon,you could put,
6 neon tetras
1 betta
or if you tank is wide,
8 pygmy cory cats
1 betta
i would go with the second choice,bettas are normally top dwellers,cories are bottom,there would probably be less agression than neontetras,my tetras used to pick on my bettas fins.you would need to add plants(live preferably).