As your research showed, goldfish are really not tank fish at all, they require a pond. I know, I had them as a child too. We could ge tthem for 10 cents at W T Grants, or they were given away, to entice you to buy a bowl etc. But they GROW. Glad you decided those are out.

Male bettas are pretty and as long as they can't reach one another, they are a good fish. You didn't specify what the sites disagree about. I kept a betta in my community tank with other nonaggressive fish and they lasted over a year. They can be kept on their own, but I never did it that way.