For your ten gallon I would suggest decorations, a gentle filter, and a heater. These are highly recommended. Snails are fine, but frogs have a different diet, and may fight with the bettas, or even eat them. I think you should get a maximum of ten bettas, but that may even be too many. If you get a male, you can either have a divider with female bettas on the other side. You cannot put two males in the same tank- unless you have a solid colored divider without holes. If two male bettas see each other, they will feel as if they always need to be alert and on guard, and this will cause stress and exhaustion which could eventually cause death. If you get females, you need at least 5 or they will fight. Don't listen to myths you hear- one gallon per betta is enough, people here will tell you that it is 5 gallons per betta, but they don't need that much. People keep their bettas in tiny bowls in flower vases, and they survive. True, the bettas will not thrive, and it is not recommended, but they will survive. But they need at least one gallon- in anything smaller the betta will be unhappy, and they will get bored and start tail biting. And it's hard to keep good water conditions in a tank that little. And bettas need a heater, and they are hard to find for bowls and vases.
The bettas: Dracula (male HM) and Mulan (female VT)
The cats: Rocky & Reginald (both male orange tabbies)
The snake: Cassey (albino corn snake, Probably female)