Wow, well done! Beautiful tank! And i too am jealous of the hairgrass!
I have 2 female bettas in my community tank (it used to be 3 but one passed when I had some kind internal parasite problem in the tank). The only aggression issues I had with them was towards male fancy guppies, so I stopped keeping guppies in that tank. So yes, it can be done. And perhaps i'm just lucky, but in smaller numbers even than 6-8, it's my second time keeping female Bettas in a community tank, last time it was 3 also.