Could a neon tetra or a guppy be kept in this tank?

ABSOLUTELY NOT! Those things are not even half a gallon. Neon tetra's need 20 gallons and groups of six. Guppies need 10 gallons and groups of three. Note even a Betta could live in that. Even Betta's need 5 gallons heated and filtered. NOTHING can live in that. Not even a shrimp or snail.