My opinion? Well, I mean, since you asked...
I've had a TON of Bettas over the years. Some were kept in 2-gallon decorative Betta tanks, but most have been in the community tanks. I've never had a Betta snap and go rogue on me. *lol* I've had some "semi-aggressive" tanks before, with things like territorial Barbs and Sharks, and have never kept them in with those. They've always been in my peaceful community tanks (Denison Barbs, Congo Tetras, Rummynose Tetra, Furcata, Plecos, Corys, etc). I've never had an issue. The one problem that I did see from time to time was in my Guppy tanks. I used to breed Guppies, and had a ton of them, and they got along fine. But if a male Guppy darted too quickly past one of the Betta's faces, the Betta would tail nip. I saw this happen a few times. The Betta would be chilling, and the Guppy would dart by and the Betta would just CHOMP. Then its brain would catch up with its mouth, and you could literally see it going, "Oh. Guppy. Oops." There was never any attacking behavior. It would nip after being startled, and then look like it felt awfully foolish about what it did, and swim off in the other direction.
I did notice, however, that every Betta seems to be an individual. I've had some feisty little jerks, and some super chill ones. It seemed like I ran into more attitude with the red strains. I've had the Bettas in smaller tanks before (10 gallons) with multiple other fish, and no issues. And I've had them in 55-gallon community tanks before, also with no issues.
I know that personality and aggression are highly heritable traits in many other animal species, so I don't know if it's the same in fish. Could be that I just got some strains with fewer jerks in the genepool. But I have had a ton of them, and no issues. The few reds I've had that have shown more spunk than usual got kept in Betta tanks. But even then, my Betta tanks always have Otos, and the Bettas haven't bothered them. Possibly because they're monochrome and spend most of their time holding still? I have had the Bettas in tanks with snails before, and they can be really annoying to the snails, but the snails would huddle down and the Bettas would get bored and swim away, and learned pretty quickly that they couldn't get to the "tasty" bits of the snails and didn't bother them much. (I raised Briggs, this could be different with other more delicate species, or those that can't fit their bodies as easily up into their shells to hide). No clue about shrimp. I raised shrimp to sell, not to feed to the Bettas, so Bettas never went into the shrimp tank. But I'm wondering now if that would be a simple way to deal with cull-grade shrimp?? Bettas are so small though, that I think they could probably only handle the baby shrimp...
Currently my Bettas are in with Otos and Corys. The first day there's a tiny bit of chasing and interest, and then they shrug and treat the fish just like part of the landscaping.
