In my home office the two Societies live in a 30" x 18" x 18" cage. The cock is flighted but the hen can only jump up on the perches though not fly across the cage. She has a few wing feathers still coming in after her moult and I hope that will make the difference. Regardless, she is the more adventuresome and outgoing of the two.
The birds have been with me at least four years and have only produced one infertile egg though with certainty Thing One is a cock and Thing Two is a hen. Having no babies has been fine with me. Both birds have imperfections I wouldn't want to past along.
A couple of months ago, it became evident something was lacking so I decided to look for another pair or two to add to the flock. In my area, there are Bronze-Winged Mannikins, Gold-Breasted Waxbills, and Red-Cheeked Cordon Bleu available though I prefer the Blue-Capped. Of course, more Societies are another possibility at least if I could find something different like a chocolate-self. There might be a crested at a shop but I haven't seen that one yet. Compatibility is more important than appearance but I'm all for avian eye-candy.

Because they will live in my office where I spend most of my time, volume or shrill sounds could be a issue. The Societies have never been a problem but the near constant chatter and flitting about of Zebras most likely would be.
So what do you think would be the best choice for a second breed of finch? Your advice is greatly appreciated.