I can understand why the original poster might look at us and wonder what's up.
@Byron and I usually agree! I still think a backed up alternative view may be useful.
Wild Bettas are not always the only fish in their habitats. First off, their populations tend to be dense - territories are close together. There are other Anabantoids (air breathing via a modified inner ear) around, although it isn't a relationship that works in aquariums. Small Bororas, fish that used to be called Rasbora, are also around. So are small barbs, not a lot of which are in the hobby. They are adapted to the tough conditions by their small size.
The region Betta splendens comes from is vast, even if we don't look at where they thrive as an invasive species. They have adapted to many many environments, with many many other creatures around. The other inhabitants don't really interact with B splendens because the niche is different. Bettas like the surface, Rasbora and barb types like the substrate. A Betta watches for bugs at the surface. Other fish are of no interest to it. The betta's territory is at the surface. Don't get surface hugging tankmates, and you're good. For certain, their behaviour is instinct, but
@Byron and I must have seen different instincts at work. We've both seen it's a shame when aquarists do things to get the Betta running all over the tank. It wants to hover under the surface waiting for bugs. If a fish gets in its way, it flares its fins. If the fish flares back, then it's on. That's why guppies, whose fins are permanently flared out, are doomed tankmates. Most other fish just swim away.
Bettas are cultivated to fight, unfortunately. The ones we get in the hobby tend to be cultivated for beauty rather than aggression, and wild Betta splendens (I've kept them twice) are fairly calm fish. Give a healthy cultivated Betta room, and all the ones I've kept have been easy going, but grumpy. A friend brought me fighting bred Bettas from Thailand once, only they thankfully were very easy going. Artificial selection doesn't always work.
There's a wealth of literature we can list, but why?
Robert J. Goldstein, Bettas, a complete pet owner's manual from Barron's , P. 49 says "Generally, livebearers, danios, South American tetras, and gouramies are safe co-habitants of the community tank."
I'd disagree with him on danios, but I know he wouldn't have minded. I doubt he was thinking of zebras, which can nip. Dr Goldstein knew his Bettas.
Helmut Pinter's Labyrinth Fish, page 96 suggests you should avoid nippy tankmates, and that's the extent of it. I don't have access to the list of sources you provided (I have read them). I just grabbed the first two books off the shelf beside my chair.
Really, all our fish need carefully chosen tankmates, and in that Bettas aren't crazily different. It just seems wrong to me to discourage people when they shouldn't be discouraged. I don't keep splendens anymore because I think the fancy fins are getting to inhumane sizes, but
@Agrigor has one and it can have a good mutually safe home with a few other fish.
Celestial pearls stay low. If you ran the tank at 24-25C, and not higher, they'd be good. So would harlequins, who have never nipped a Betta in any of my tanks. Corydoras breathe air in the mouth and out the butt, and their dashes to the surface for air will sometimes take them smack through a Betta's turf. Bettas don't like that.
