Thanks everyone for your help. We did a little flaring exercise today...Zebra is indeed a girl. Her flare is so minor!
She went back in with her sisters in the 20 gal - did a massive rescape of it and added them all back in a the same time. No aggression at all. These sisters are just so sweet to each other.
I think I am going to be stuck with a sorority tank. Is five giants too much for a 20 gallon? I'm going to try to give away two of my seven, because I think seven is too many.
IDK if I could have a female alone in a tank. My male is just fine in his own tank, but he's with us in the kitchen. It wouldn't be right for me to leave Zebra in the 5 gallon all alone in the fish room...she'd be lonely, especially since she and her sisters seem to like each other so much, and hang out together constantly.
And, I've learned that I hate small(5 gal and less) tanks. This is just IMO, but they are simply too small and cramped, even for a giant betta. I tried it three times now, and it just doesn't feel right to me.