I did this for awhile because I needed to, but my king quail were stressed out by the intrusion (it was their aviary after all). They did not nest like they had been, and they weren't as relaxed. I also had to remove one of my Japanese quail hens because she did not like those little quail and she was going to have a piece of them. So it isn't just the male you need to worry about. Japanese hens can be just as aggressive if they take a dislike to another bird. And there's always the risk that a male Japanese quail will try and mate with the king hens.
If you hatched out and brought the two species up together and only had Japanese quail hens with the kings then that might work, but the two species have quite different personalities and because your Japanese quail aren't used to the other birds they may attack them.
