There are times bettas can be with fish like tetras and whatnot... However... You have to be careful and always have a backup plan. I got two "semi aggressive" gouramis, and haiwaii platys. Those darn platys pestered my poor gouramis to death! And the gouramis were twice their size....... -.-; THAT right there proves how dangerous it can get. I have had regular platys before and loved them. These ones? Nuh uh! They found a home so I could make a sorority instead. (In the sorority three plakats were removed for violence x.x). Bettas, are by far one of the hardest fish to find the compatability for. Especially when they have their own individual desires, and quirks.
Spartan was fine with most fish, as long as they did not pester him. This included danios, who learned he was boss and left him alone. But would I try that again? Probably not...
Mainly because they take up the same water space. Top level. Bettas tend to stay near the top but travel all over.
Best matches: Bottom dwellers. African dwarf frogs, cories, and shrimp. However, cories tend to love more space... Longer the better. Besides their bioload, they are skittish, and can cause harm to themselves by panicking... and do best in groups of 4+. I'd love to get some again!! ADFs are nice, and again better in numbers (socializing). I have had one munch the fins of my betta though... x.x Shrimp, are usually delicious snacks!
But, it depends on your betta :3 If you can, try getting a 20-30 gallon, to hold the bioload of what you have, plus the extra few cories you get. Easier on you and them :)
Forgot to add... I treat adding new fish like adding a female to the sorority. I rearrange the decor during a water change, may or may not place the betta in a breeder's box/net which is not always necessary.