These people are wrong, unless....
The tank is too big, i.e. 30-50+ gallon tanks
With filtration that is too powerful
With fin-nipping fish
They should be ok if the tank isn't too big, if there's only one male, if there are no female bettas, and iof the other fish are docile. Suitable companions are ADF, ghost shrimp, cories, most livebearers. They may try to mate with guppies, so you might want to be careful.
Don't add them with tiger barbs or neon tetras. Not sure about danios, though.
