You need one more nest than you have pairs. So if there are 2 pairs, you need 3 nests (minimum). Also do you know about the height thing with finches? They don't like to be in the lower nest, so try to put them all at the same height, but not facing directly into another nest. If they do have to face each other, you can hang fake vines or flowers between them as a barrier.
Your zebras WILL breed with society finches and I think some other types, so it's best to stick with the species you have now.
Once your zebras have babies, the young can breed as early as 12 weeks -- and you don't want them cross breeding with the parents. So have a plan in place as to how to separate them.
That's all I can think of at the moment. Good luck. Be careful breeding zebs, as they multiply really fast and people get overwhelmed from them. Don't be afraid to pull the nests and toss out eggs from feeders, etc. in order to keep from being overrun.
