Are all the tetras schooling together well? I love your stocking choices also. I would personally not go with a Betta because sometimes even the females can be finicky and some fish are dicks and will fuck with their fins. A small school of diamonds would look awesome. I would keep it small though as it might get a little overstocked. Maybe not though. Bettas don't really flair their fins much anyways so a lot of the times they can look pretty boring. Maybe an angel? Could look interesting.