Honestly, I keep my wild bettas in smaller quarters than recommended and have no problems. I house a unimaculata pair in a 10 gallon tank and they just gave me 20 odd fry. I have a group of six brownorum (3 pairs) in a 12 inch cube and the dominant pair spawns weekly.
I've found as long as you offer plenty of cover (java moss is the best for wild bettas I've found) and maintain correct water parameters, you can usually have them in smaller accommodation than what sources online recommend.
Imbellis might squabble a bit but I had a group of four and really it never went beyond body slapping and chasing. They are much less aggressive than their domesticated counterparts.