Diandpat, I really think you need to let them be for the most part. Remember, if you get stressed, they will pick up on that. If Zuzu is trying to get away and hide and Ziggy keeps after her, then give them a time out from each other. I find if I leave Jasper and Sweet Pea alone they sort it out and become best friends in a few minutes. Sweet Pea adores Jasper even if he plays too rough with her (or so she makes it out to be, so Jasper eases up, then she goes in for the attack
I have seen over and over my foster kittens doing what I think is so painful to each other
but they love it
and keep doing the rough play over and over again.