beeps and peeps
Hi, and welcome to the Forum.
Zebras, will be Zebras..... they are the feisty Terriers of the Finch world. Most everyone who has Zebras, has had trouble with Zebras. Either the boys are fighting, or the girls, are trying to kill each other, or someone is plucking everybody, or....
You could try another pair, but there are no guarantees that all three pairs won't go at it.
My recommendation, would be, to create different little areas in your flight, with artificial plants, flowers, vines, twigs, braches, etc. This offers the birds private little nooks to go into, to get away from whomever happens to be on the rampage.
Zebras, being territorial, will tend to adopt certain areas in the flight as 'their own'.
Will that solve all the aggression problems? Probably not, but it should help a bit. I would also offer some swings, and a foraging box, to give them something else to focus on rather then just each other.
In the long run, they will simply have to figure out the pecking order for themselves (as to who the top Zebra is ), and then things should settle down.... for a while anyway
Good luck.