If guppies and plecos are there, then they are there as invasives. It's a worldwide problem.
Just want to write this msg to say totally agree with you. The biggest issues I have when am trying to give someone advice u give it and then they come back to you and say well the fish experts in the shops says different i do firmly believe they are the problem big companies just want u back for more businessGiven the objects/plants you want, yes, a 20 gallon would be very much appreciated by your Betta.
Read my signature.
First of all, a minimum is just that. It's not optimal. People are tribal. They like to go with the crowd. Most people are simply ignorant and simply repeat what others have told them. That's a statistical reality, demonstrated quite clearly by the Internet and social media.
I know fish sellers that keep Bettas in one liter jars. Just because those fish can survive doesn't mean it's a good life. How would you like to be kept in an 8'x8' cell? As long as someone provides food and water, you'll survive. Is that your idea of a good life?