I do not want to be arguing over this, again (a couple of similar threads recently), but this is only a part of the issue. This will explain the issues and why it is a serious risk to the fish (betta and the others).
Betta splendens seems to live solitary in its natural habitat which is still and sluggish waters, including rice paddies, swamps, roadside ditches, streams and ponds. Such an environment is not conducive to fish that require oxygenated waters so one can expect few if any non-anabantid species to live in such habitats. During the dry season, most bettas are able to bury themselves in the bottom of their dried up habitat. There, they can live in moist cavities until water once again fills the depression during a rainy period. The fish can survive even if thick, clay mud is all that is left of the water. They do not survive total drying out of the bottom (Vierke 1988). There are very few fish species, and none that are found in the same habitats, that can manage life in such conditions, which is further evidence that the B. splendens is most likely a solitary species.
All anabantids are territorial; male bettas instinctively fight each other in defending their territory. Selective breeding over many years has produced fish with a heightened sense of territory defense, which explains the common name of Siamese Fighting Fish. Fish fights for money is a "sport," if you want to use the term for such animal cruelty. This means the bettas we see in stores have an even greater propensity to literally kill each other given the chance. For a fish that instinctively lives alone, and believes it must defend its territory to survive--both traits that are in the genetic blueprint (DNA) for this species--this aggressiveness is likely to extend to any fish that dares enter the betta's territory, which in most cases will be the tank space. And forcing the fish to "live" under such conditions is frankly inhumane.
Individual fish within a species do not always adhere to the "norm" for the species; this is true of all animals, including humans. But with fish, responsible aquarists should research the fish's behaviours, traits, and requirements, and then aim to provide accordingly. "Expectations" are as I said above programmed into the species' DNA, and we are not going to change them just because we may want to have a betta in the tank with "x" fish species. Sometimes the betta seems to co-operate with our experiment, but in many of these situations it may not last for long, eventually if not immediately. Fish that do succumb are likely being severely stressed, unseen to the aquarist until it is too late.
If the betta does not first attack the intruders, the intruders may go after the betta. It is a two-way street, and in either situation it is the betta that loses in the end. Severe stress causing increased aggression, or conversely severe withdrawal from being targeted by the other fish. And physical aggression is not the only concern; fish release pheromones and allomones, chemical communication signals that other fish read, and these can cause stress and promote aggression that will in time weaken the fish to the point of death. There is no reason to risk the fish in one's attempt to prove scientific understanding wrong.
References:
Betta splendens profile on Seriously Fish.com
Hargrove, M. (1999), The Betta: an Owner's Guide to a Happy Healthy Fish, Howell Book House.
Kottelat, M. (2013) "The fishes of the inland waters of southeast Asia: a catalogue and core bibliography of the fishes known to occur in freshwaters, mangroves and estuaries," Raffles Bulletin of Zoology Supplement No. 27: 1-663.
Tan, H. H. and P. K. L. Ng (2005), "The fighting fishes (Teleostei: Osphronemidae: genus Betta) of Singapore, Malaysia and Brunei," Raffles Bulletin of Zoology, Supplement No. 13, pp. 43-99.
Tan, H. H. and P. K. L. Ng (2005), "The labyrinth fishes (Teleostei: Anabanatoidei, Channoidei) of Sumatra, Indonesia," Raffles Bulletin of Zoology, Supplement No. 13, pp. 115-138.
Vierke, J. (1988), Bettas, Gouramis, and Other Anabantoids, T.F.H. Publication, Inc.
