I haven't done it personally, but I've done a fair bit of reading on sororities, because I think they're really cool and I love bettas! They can get very territorial, so make sure that you have lots of different hiding places where they can get out of view of each other. They will establish a pecking order and pick out their own territories. Rather than adding another type of fish, I would suggest doing more female bettas. The more you have, the less likely they are to team up and kill one, which is definitely a possibility. Try to pick out fish that are similar in size, and make sure you add them all at the same time. And hope you don't get a really aggressive female, or you could be in trouble! If you end up adding new fish to the tank at any point, take all of the fish out and rearrange the tank so that no one has an established territory to defend.
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