Hm... It's too small for Tetras, sorry. :( You could only put in Snails or Shrimp. Tetras are really active fish and needs schools or they'll feel insecure and shy. You could put in like Red Cherry Shrimp, Ghost Shrimp, Nerite Snails, I think Mystery Snails are OK in an almost 8 gallon. If you're going for Shrimp, remember to have lots of plants so they will have enough cover to live from your Betta. Bettas love Shrimp. Especially mine. He ate all 4 under 2 weeks.
Over hundreds of bettas die in their small, dirty cups each day. Only you can help them live. Research and then save them, today!