Betta are nice but it would be handy to know your dimensions as alot of hexagonal tanks are quite deep and not very wide which doesnt really suit betta's.
You could go for a small community of tiny tropical fish like microrasboras, small snails and shrimps and maybe some wrestling halwbeaks or some guppies.
the thing with heagonal tanks is you need to keep fish that like different water levels.
if going for a betta why not look into something other than betta splenens. there are some really naturaly beautiful betta out there and if you no the right person and have a dedicated tank there is no reason y you should need to stick with b splendens. besides most other betta's dont want to kill other males so a group can live happily.
Look at betta coccina, betta channoides, and betta imbellis. for the first two you will need quite soft water though and idealy they all prefer shallower tanks with a larger foot print as they all like to take atmospheric air and need to be closish to the water surface at all times.
i dedicated species tank for dwarf puffers is a nice idea
