I've been researching this for about a week and from what I understand it really depends on our Betta (marlin) if he even prefers tank mates or not. We've had him about 2 weeks and just upgraded him from a 1 gallon to a 10 gallon after reading more about what would be best for him. He seems to be a very happy, easy-going fishy.
Our plan is to get about 5 ghost shrimp to start and monitor how Marlin feels about them and whether or not they become lunch. However, I'd really like to have about 6 tetras in the tank as well if the ghost shrimp work out. I've read that neon tetras might not work out because they are too hyper and might nip Marlin's fins.
What types of tetras are best as tank mates or are there better fish that I'm not aware of?