If you mix DIMMs with different frequencies and timings, you have to use the slowest clock and slowest latency timing of the modules you are mixing to maximize your chances of getting something that works.
Ex.: if you are mixxing 1866-10-10-10 with 1600-9-9-9, your best shot would be something like 1600-10-10-10. If this does not work, you can try 1333-10-10-10. If this still does not work, you are likely out of luck.