Six females in a 10 gallon is fine. Female bettas are like cichlids, you want to overstock as it disperses aggression and stops individual fish from being targeted. The only downside to this, is that water quality is paramount, and you need a filter with plenty of room for biological filtration.
You also definitely want to have as much cover as possible. This should cover all levels of the tank. I find tall plants, whether fake or live a much better alternative to caves and other artificial decorations.