I've looked at a lot of articles where betta behavior has been studied. It's almost always proved that bettas are NOT territorial. In fact they will fight regardless of how much space you give them. The only luck with a larger space is that they won't see each other as much, but as soon as they see each other - it will be endless fighting until something distracts. You could say that a betta's territory is the entire body of water it lives in. They don't fight each other to defend their water, they just simply fight.
I have noticed with my bettas that generally they will mostly fight with others that look most dissimilar in appearance of color and fins. If I put my cambodian (with red fins) together with my totally red female. They dont usually fight. If I add a blue/black betta to the group then the red one will mostly go for her. Maybe this is a result of them being raised together with other similar females until being sold.
I need to get me some docile females
some behavioral traits can be bred ^^