My male betta being in my community tank was experimental, and I've had a customer get crazy on me about it, but like others have said... it all depends on the fish. That's PLURAL! Some fish, both large and small, can be aggressive.
Example: I had a few GloFish (tm) in my community tank that did fine and never showed aggression toward any other fish, but the shoal got smaller and smaller as they died natural deaths, and I was left with one. When he was the only one of his kind left, I saw him start taking shots at other fish in the tank. I quickly bought him some new friends to school with and the aggressive behavior stopped. That said, as pet owners we are responsible for maintaining the balance for our pets because they have no choice or means to do it themselves.
Fish are like people to some degree. They act differently when they are alone than they do when there is a group around them of like kind or when their environment changes. Logic would tell us that bettas in the wild can't always be alone, or they wouldn't have survived as a species. In the wild they have plants and hardscape around them naturally to house and protect them. Plastic and glass are not natural! We can't really eliminate the glass, but we can eliminate the plastic and provide them with real plants and hardscape to comfort them and help make their glass houses a home. We took them out of nature, so the best we can do for them is to provide a home that is as natural as it can be within our capabilities. I get a lot of flack for that opinion, but I'm not sure why.