These tetras share the same water conditions as bettas, low pH, high temp, and soft water. However their colors may cause the betta to become aggressive. Their speed though counters this problem in that they can easily avoid the bettas attack. Additionally, tetras enjoy being in schools so 5+ in a 10gallon tank is necessary for healthy tetras. Make sure to have plenty of hiding places for any fish to escape to.
Plecos (not common pleco)
Plecos make excellent algae eaters but questionable betta tankmates. The only reason why they are are questionable is because of the tank size. Common plecos can get up to 2 feet, the length of a 10 gallon tank. And since most betta owners don't keep their betta in an aquarium more than 10 gallons, common plecos are a no. However, there are some smaller species of plecos. For example, clown plecos, bristlenose plecos, pit bull, and rubber lipped plecos max out at around 5 inches. So one can fit in a 10 gallon tank. Additionally, the armor plating on these algae eaters will protect themselves from any betta aggression.
If you really want to have guppies with your betta, your best bet is to get feeder guppies. Although really inexspensive, they are dull colored so they won't look as vibrant as fancy guppies. This is why the can be housed with bettas. However, because they are sold as feeder fish, many of them will be sick and can transmit whatever disease they have to the betta. As long as you get healthy fish, they should be good tankmates.
Rasboras make good possible tankmates for the same reasons as neon tetras. They enjoy soft water and low pH. And similar to neon tetras, rasboras enjoy being in large groups. One difference between rasboras and tetras is that rasboras are more peaceful than tetras. However, their colors are less virant than those of a neon tetra.