The only way to get sexed guineas is by buying adults locally. It is preferable to have an equal number of males to females but is not a requirement. Nearly the same age guineas would probably work best. Since yours is a male dominant flock, it should be fairly simple to integrate a mostly female group.And thus I return to the guineas... if I buy 6 more to give them their preferred flock of 10, I'm assuming I have to have 5 males and 5 females. Since I already have 3 males and 1 female I would need to get an additional 2 males and 4 females. I did not encounter a seller who sexed the guineas so I'm not sure- how do I guarantee I get the ratio I need? Also, these guinea guys are now 5 months old. What age guineas am I looking for to keep aggression between everyone at a minimum? I'm really happy if Gracie will survive this experience without losing her mind so I want to get them as soon as I can- but the RIGHT ones. Also, we live in the mountains and it is going to be below freezing for good starting next week. This institutes the other issue of no one being able to get outside their coops <sigh>.
Back to your other issue, the sooner that you get your chicken out of there, the better off your guineas will be. From your description, your chicken is the alpha and she appears to hate all other females. Your male guineas are responding to her behavior instead of having normal guinea behavior which is why they are abnormally picking on your guinea hen. I would tell you what I would do with that hen but I am sure you don't want to hear that kind of a suggestion. If you don't have anywhere else to put her, at least try rehoming her.