How should I go about re stocking a 20 gallon?

So my grandma has recruited me on re stocking her fish tank for her. I only really have experience with my bettas and some neon tetras. The tank has not been stocked in years and the fish have just died off. At the moment there is only 1 neon tetra and 1 black skirt tetra. I know i need to buy 4 more neons and I am not going to buy any more black skirts. I would like to get docile male betta to put in the tank. I know how to tell if a betta is docile. What are some other options I could go with?