Real plants shouldn't have leaves sharp enough to tear fins, but I thought I saw some fake decor in the photo? Some of those plastic ornaments, or even real wood, can have surprisingly sharp edges, so worth checking and sanding down any that feel sharp when you run your finger over them.
Have a read through this
paper about bettas, it contains lots of useful information, including a part about bettas who showed a lot of flaring and aggressive posturing when exposed to seeing other bettas, stopped showing as many aggressive threat displays after continuous exposure to the other betta. But the note about that that I find concerning is:
"The question is whether it concerns actual habituation to the sight of the other male betta, or whether other mechanisms, similar to learned helplessness, underlie these findings. Although threat signals may wane following prolonged exposure of male bettas to each other, this does not seem to influence fight readiness or fight outcome (Meliska and Meliska, 1976; Meliska et al., 1980)"
Now consider this;
"When multiple fish species are kept together, behavioral changes are seen both at interspecies and at species level (Sloman et al., 2011). It has also been documented that male bettas send aggressive signs to other fish species, which may be followed by an actual fight (Johnson and Peeke, 1972; Johnson and Johnson, 1973; Miley and Burack, 1977)."
All fish release hormones into the water which sends signals to other fish. Pheromones and allomones. You'll need to do your own research to find out more about that. But knowing that, I personally wouldn't keep a fiercely solitary and territorial species like a betta with other fish, both for the betta's sake and the other fish. If the fish seems chill, I can't prove whether he's actually okay with the other fish, or he's suffering from learned helplessness; because he certainly isn't given a choice in the matter, we put the fish in these situations.
I know for sure that bettas live solitary lives in the wild, males only interacting with a female briefly to spawn, before chasing her off as well. I know that captive bettas can and will attack other species, seeing them as invading their territory. So despite some personality differences, a betta is still a betta, and we can't and shouldn't try to change the nature of a species that has evolved to live in a certain way for thousands of years. I wouldn't want to risk the betta snapping and killing the neons.
But whether you choose to do so or not is your decision. But at least now you have some info you can look into, so you can make a fully informed decision.
