If you actually have 3 female bettas that are not flaring and hiding/chasing throughout the day, and they have already been together for quite some time, I wouldn't mess with that by adding more bettas. There's a very small chance you'd end up with more that would fit in so nicely..as it's already quite lucky if you have the three doing well long term. Even if they are doing well, it's still likely it will end in disaster. IMO female bettas can surivive together but I'm not convinced they can thrive. I split up my sorority of females after several months after coming to this conclusion.
That being said, if they do well with your rams I would speculate they will be fine with the apistos.