It's pretty much the golden rule to never put two bettas together. Ever. (Unless you have a divider) There are ways to keep multiple females, but not in that tank and not with such little personal experience. If you wanted a tank mate for your one betta then I would suggest a couple of ghost shrimp. Otos need to be in groups and are sensitive to water conditions, so I wouldn't add any. Snails produce a lot of waste, especially compared to shrimp.