I think with the greatest of respect, that you need to educate yourself a little more.
Betta fish are naturally aggressive. Thats because in their natural habitat they are top of the food chain. Just because you take the fish out of its wild environment doesn't mean it changes its natural instinct to dominate and claim territory by fighting sometimes to the death. And by its own natural right, it wants to dominate anything and everything.
Putting a Betta with anything else will cause it a great deal of stress because of it's fierce natural instinct to protect and defend its territory. They are highly strung fish with a major attitude problem, that don't want to share or compromise with anyone else. Imagine being in a constant state of fight or flight, high adrenaline and stress. Sound like fun? It takes a massive toll on the Betta's health and drastically weakens their immune system leading to premature death.
Betta fish are not schooling fish, even in a big aquarium.
And just for your information, neither are Dwarf Gourami!
There are serious errors in this post. Bettas are the top of no food chain. They eat mosquities, and everything eats them. There are predatory insects in their environment that pick young Bettas off. That's why they are shy. They are targets. They defend feeding and breeding territories against other male splendens. Ideally, they get to do this if a bird doesn't pick them off.
Their survival strength is the labyrinth organ, that allows them to colonize low oxygen swampy zones larger predators avoid, because in shallow water, birds have a clear view. Larger predators get predated. They share with some tiny Bororas types, who handle that environment by staying small. But pet shop Bettas were removed from that environment hundreds of years ago, and cultivated for gambling. They've been jarred for centuries (not good in my eyes, btw - just a fact).
If you read the thread, which it appears you didn't, I have kept wild caught splendens. They were relatively unaggressive. I have also watched go pro footage of them, taken by a forester friend. They hover , sometimes with other males in sight. They look up, for bugs. They conserve their energy. Conflict is fin flaring, and the invisible barriers are generally respected. You'd need a four foot by four foot one foot deep sandbox tank thick with Pistia or water hyacinth to see this
if you had wild caughts not bred for gambling or mild plakats raised together to see this at home. I've only ever seen one tank like that, run by a friend who had my plakat young in it. They had grown up together, as I had handed them over at under an inch. The tank ran for several years as a real conversation starter.
There's a hobby folklore/fishlore that says these fish are mad killers. Put them in a 5 with other fish, and yes, they can be. Most males though will take a zone and stay in it, and posters will ask why their Betta or 'beta' isn't at the front of the tank all the time. Are they better alone? With the bad breeding that creates the oversized fins, the fish is visibly handicapped, and may not be able to thrive in a community. I will never buy another longfin as my experience with wild caughts convinced me keeping fancy Bettas isn't ethical.
Hundreds of years of selective breeding created fighting lines for gambling. That's a problem. The percentage of "good fighters" was always low, which is a good thing. Decades of hobby breeding created fins that slow them down, and a Betta usually learns to live and let live quickly. When you buy them, they have been jarred since very early, as soon as their sex could be guessed at. This is to protect their fins because if they squabble, they tear them and we don't buy them then. It also increases aggro temporarily, as they have never shared space. Most Bettas adjust in a day or two. Then they are grumpy things, not stressed wannabe serial killers.
My sample is about 35 Bettas over 56 years of fishkeeping, with a another few hundred that I raised and sold or gave away when they were unsexable juvies. Lifespans for single Bettas in communities ran at about 5, although one got to 7 having lived in both types of setups. I used no tiny tanks except when I was a kid starting out and not knowing better. Even those fish were chill though. Not one of those bettas ever killed a tankmate. They barely chased them after 2 or 3 days in the tank.
In a properly chosen community, they tend to be the victim of fin nippers, not the killer. Some will show the breeding for gamblers still, and are usually the ones fighting with the reflections all around them. Avoid those ones.
We all need to constantly educate ourselves more, and I read every paper I can get on the Betta group, even though I don't plan to keep them again. If I were to stumble across a coccina group one, or picta - yeah, I'd bite. No more splendens for me. The issue isn't their aggression, it's ours. We feel free to create misshapen fish bred for aggression since some of us like to gamble on death. We have screwed a cool fish up outrageously. But in large enough tanks here with carefully chosen tankmates, they seemed very unstressed, long lived and calm.