A single betta would be a perfect choice for this tank. You cannot have 2 bettas together as they will kill each other.

Good for you for following all the great advice you've been given. I can't believe someone at a fish store would have sold you all those fish for that little tank. Shame on them.

But again - good for you. Return the fish asap. start your fishless cycle, get a heater and a larger filter and soon you'll have a wonderful environment for a betta who will give you hours of pleasure.