I would consider adding more plants and hides. One problem with a sorority is that the interior decorator in you goes straight out the window. You want heavily planted and lots of hides, to the point that it can look cluttered.
Your stressed girl, is she smaller than the others? Or is she near the top in size and trying to be the alpha female? If she is too much smaller than the others, you have to consider putting her in solitary until she grows more.
I recently lost my beta female because she kept picking fights with the bigger girl and trying to be alpha. Found her one morning gutted and missing both eyes. In hindsight I wish I would have just removed her.
Lastly, when you get ready to add that last pair, I recommend the following. Float them in their bags, acclimate to water as usual. When you get ready to release them, get everything ready, then throw a bunch of food in the water at one end of the tank, and release the new girls at the opposite end of the tank. Keep your existing girls distracted with food for a while. Drag it out, maybe even over feed just a BIT. This worked great last time I tried it. The existing residents barely noticed the new comers by the time they were done stuffing their faces, and the new comers had enough time to explore without being attacked.