I recomend tetras over guppies and they exist at all levels of the tank, they are so much fun (not neons, they are boring imo lol) but I have 4 phantoms in my tank (1 male, 3 girls) and they are very fun and interactive with the betta mostly cause they are terrified of him. :D Flame Tetras, or Von Rio tetras are also good, just don't get Black Skirts, "Serpai" cause they get HUGE even if it says 2", I saw a 4" one in the tank at petco. or anything from Petco that says "fin nipper" on it. (Rasboras and Platies are good too!) You have tons of options, your tank size is the limit.