The two charcoal males returned to the aviary without issue (although the smaller of the two seemed to have poor flying stamina initially). I returned the pair of fawn Zebra Finches (nothing eventuated from the seven eggs they laid) and it turns out the male is quite the bully. He insisted on occupying a large perch and would let no other bird near it. The male grey Zebra Finch in the aviary is a solid little bird and even he was being pushed around. I was hearing squawks I never thought finches could produce. This was not good.
So late today I removed the pair of fawn Zebra Finches and placed them in a small cage indoors. The atmosphere in the aviary changed instantly. When I have cleaned out their cage they can go back in there.