I never done it or seen it but I don't think the Royal will go after them. The biggest concern would be the water conditions...make sure they match(ph, hardness, temps).
they do fine with discus, but the poop they give will be a problem. I had to move my 4'' l191 out of my discus tank because it is pooping way more than all 8 of my discus combine. And it is so hard to keep the tank clean of poop without a strong flow filter which discus hate.
I think it's possible to have high filtration and discus. Just point the water flow towards the corner to facilitate siphoning up poop.
I think the bigger problem is accidentally feeding royals too much meaty foods (and develop bloat) that discus like to eat.