Once again Emily is spot on. You need a ratio of 3 females to 1 male when it comes to guppies. The tetras are schooling fish and you should have at least 6 in a school. That means picking one, black neon or neon. Instead of the guppies you could get a couple of corydoras catfish. That way you'd have a mid level group (school of tetras) and bottom feeders (catfish). Having two levels adds interest and balance to your aquarium.