the 200% XP gain cap is exclusive too the legendary and mythstone XP bonuses all other XP bonuses will work with it 
so like the 175% bonus from mythic 3 will stack above that cap so go ahead and put the quest mythstones on the build and going into the future you should check here Griffin’s Smithery: Masterpost first it will most likely answer whatever questions you have (not nessacarily yet but when I’m finished all of the guides) or ask these questions on the question thread hope that helps 