Never a good idea to mix DRAM, it can be problematic, but it's your call...XMP won't work as the programming is by the packaged set and in the advanced timings the settings differ from 2 sticks to 4, your 2x8GB might call for a tRFC of 178 or so and the 4 stick set may want like 255, so you may have to hit or miss on setting up the timings