I'd scratch out otos too. They need a group to be happy, they're a schooling fish that live in groups of thousands in the wild, and they're all wild caught, so keeping a group of at least six is the least we can do. Some people do keep just one or two purely to use as tank cleaners, but that's unfair to the otos. They're also delicate fish and sensitive to any changes in water parameters, and need a good supply of algae and biofilm to eat, so best to only add them to a very established, well planted tank, so would need to wait until the tank has been up and running for a few months before adding them, be sure you've got a handle on keeping ammonia and nitrites at zero, and nitrates below 20ppm, and better if it's a well planted tank.
I've never kept bettas so can't help there, I'd only go for a betta sorority though if you're a very experienced fishkeeper and can be sure you'll be able to tell when there's a problem and remove any trouble makers before it gets bad. From what I've read, there's no guarantees, and many people who set up a sorority have to try several different females before they find a balance that works, so you'd need spare tanks in case a problem arises, so you can remove a fish before it gets killed. To me, they're not worth the risk since it's for the keepers benefit, and not how these fish would live in the wild. I'd personally go for a regular community tank, sororities sound like a lot of stress for the fish and for the keeper, with a lot of potential to go wrong.