Except for Zebras and Societies, most finches don't breed well in a community cage, unless it is a large flight. I don't have an aviary, so any finches I want to breed I put in breeding cages, 1 pair to a cage. My one pair of Owls are very flighty and quick to come off the nest, so they are on the top tier of shelves, plastic plants around, try to give them privacy. If you put one nest in with 2 Zebras and a pair of Owls, the Zebras will most likely take over the nest. If you put in a second nest, the Zebras may take that one, too. If you add a pair of Societies to foster, the Zebras may take over that nest, too. Could you put a pair of Owls in the cage where you kept the Zebras? Or if you get Societies, could you put the Zebras back in the original cage and let the Owls and Societies have the flight? As far as other fosters, I have read that Zebras have been used to foster Cordon Bleus, and I have also read that African Silverbills make good fosters.
I have 5 Gouldians in that HQ cage, 2 males and 3 females, and they are sitting on eggs (now using two different nests). They have all been together for months in a similar sized cage, and got along beautifully, and even with them all in breeding mode, there hasn't been any fighting--some of them are even sharing nest duties! Now, this is not a great idea, because I will have no idea of parentage of the babies, but it shows that there are exceptions to the rules--5 birds, 2m/3f, and they act like they are in a community aviary setting. I think it is working because Gouldians are so passive.
What I am being so long-winded about is that there really are few hard and fast rules, just guidelines, as each finch is different. Sometimes you just have to try things out and see if they will work, but then you always have to have a back-up plan, extra cages ready, in case it doesn't. Case in point, I acquired a Strawberry hen and a Shaft-tail cock that were being rehomed. I thought I would put the hen in with my Strawberry cock, since his mate has shown no interest in nests last year or this year. Well, the new hen hated the Strawberry cock, chased him unmercifully, and I finally gave up and put her back with the Shaft-tail. She is in love with the Shaft-tail and is happy as can be now. The one thing you can't do with finches is force them to do what you want.
So, you could try the Owl/Zebra combination and see if it works, if you want. They could be the exception to the rule! Well, have I confused you enough now?
