I do have a leather coral, so I thought the same. But most of the softies are not aggressive. Now potassium. I haven’t tested for that but I also assume since I make water changes it should be ok.In my experience no light is too bright for montipora, especially plating/encrusting.. Same applies to flow - no flow is too high.
I wonder if you are having some other issues, such as water quality and/or the fact that it's a softie tank (and so the chemicals released by soft corals could be negatively impacting the SPS). Also check potassium levels - plating montipora needs it badly.