1. The tetras shouldbe okay, but that's ALL you'd be able to put in there. They need schools of about 5 to feel secure, and 5 in a 5g is pushing it.
2. Not sure that I'd put more than one gourami in a tank that small
3. Definitely no Betta/Gourami mixes.. they tend to fight
You should stick with some smaller fish such as neon tetras, black skirt tetras, or a betta and a snail. Bettas are very easy to care for and are very forgiving of mistakes. The snail you'd need to keep a close eye on and make sure that NO copper at ALL makes it into the water. (Copper is very lethal to snails, among almost all invertebrates).