Personally i wouldnt put any fish no matter how small into a tank of less than 10 gallons on a permament basis and even a 10 gallon tank only offers a floor area of 18x12" (1.5 square feet). 10 litres is the volume that the average household bucket holds and im sure if someone said they were keeping fish in a bucket there would be a few choice words said.
Then I guess we have a difference of opinion, that I of course respect.
I agree, that if one admitted to keeping fish in a bucket then something might be said. But I suspect this is probably because of the image of no filtration, heating, decor etc. that one possibly visualises with a "fish in a bucket". Volume, I suspect would be less of a concern is the fish size/species and equipment were appropriate to 10 litres.
You may have already seen my 10 litre aquarium. I think the tank size is quite suitable for my fish, in the long-term too. Many experts agree. If I suspected the fish were not suited to such a small volume then I would of course re-home them in a larger tank. Although I am a plant "nut", my top priority is the health and well-being of my fish.
It is worth re-iterating that this size is not suited to all levels of fishkeeper. I perform 50% water changes every other day, the filter is effective, and the plants themselves are great water purifiers. Oxygen production is massive. There's plenty of swimming space for the tiny fish (16mm adult according to Fishbase). The hairgrass provides security, and combined with riccia are both excellent spawning mops. All in all an ideal environment for the inhabitants I would like to think.
I'll leave the betta issue to the betta crew.