Hi and congratulations on your new fishies, I applaud your efforts to try to do right by them. I'm not an expert but I though I'd mention my experience with swim bladder issues in case it helps you.
When I first got Gandalf, (my avatar) he had serious swim bladder issues. I'd give him one tiny pellet soaked in water and a couple hours later he'd be so puffed up he would bob to the top on his side and not be able to swim at all. It was terrible. I tried all the tricks, fasting, the pea trick, I spent days on the internet reading and reading trying to help him. My heater was not adjustable and the box said it would only keep the water at 78, I could not get the temp higher then that. So I bought a new heater that would go up to 90 and set it for 82 and he loves it. His swim bladder cleared up immediately and I haven't had any issues since then. I'm not saying this will fix your betta's swim bladder issues because all bettas are different,, but it seems to have made a huge difference for my betta. I've read that their metabolism is very slow and will slow down even further if they are cold, so sometimes that contributes to the swim bladder issues.
Anyway, that was the long way of saying maybe a heater will help your little guy that seems to have swim bladder issues.
PS, the 6.6 "PETCO bookshelf tank" pictured above is what Gandalf is in. I love the tank and would buy another in a heartbeat.