You should add at least 3 or 4 more female bettas in there. Then you would still have room for two small plecs. Just realize that they create a LOT of waste, and lots of water changes will be essential.
One small female bn would fit, but in my opinion there isn't enough room for two, as they are very territorial. If you have a fine sand substrate, kuhli loaches or a school of corys would be nice.