It should run together fine. It may default to DDR3 1333 cas 9. Since the cas 7 sticks are 1.6v they will probably run at DDr3 1600 cas 9 just fine at 1.5v. You may have to tweak the voltages and timings a bit to find a happy medium. I would try DDR3 1600 cas 9 at 1.5v for all the sticks. You might even go to 1.55v and be able to run it all at DDR3 1600 cas 8. GSkill RAM is known for being great at overclocking both speed and timings. If you were mixing brands and speeds you might have issues but these are probably close enough. Stick size does not matter a bit when mixing.