It's a pattern that far too many people new to the hobby seem to have. They don't ask to get a logical, thought out answer from experienced aquarists. They ask because they're hoping to find that one other person that's willing to agree with them, so they can justify doing what they already know is wrong. They want permission to "break the rules."
Betta in big tank = unhappy betta
Betta + Angel = bad news, potentially life threatening
Betta + Betta = bad news, potentially life threatening
Betta + Gourami = bad news, potentially life threatening
This is what I have learned through research and through the shared knowledge and experience of fishkeepers far more knowledgeable than myself.
I understand the mentality of needing to learn everything from experience, trying things to see if they work, etc. I really do. The world would never move forward if we didn't deviate from "the norm" from time to time. But when it involves the lives of living creatures that a person has assumed the responsibility of keeping healthy, happy, and safe, ignoring advice and warnings from those with more experience can be extremely harmful, and goes to show how much a person actually cares about their pets. Half the reason for forums like this in the first place, is so that people can learn from the mistakes of others, without having to make them themselves.