If you use 1600 sticks and 1866 sticks it should down clock the 1866 automatically to 1600. Running 2x8GB kit and a 2x4GB kit will work fine, you just have to make sure that you setup them correctly.
So what you basicly do is set up the 2x4gb kit in it's own dual channel and 2x8gb in it's own dual channel. 8-4-8-4 do put it simply. It should work, but you have to check how you have set up your BIOS profiles. You might have to manually clock the 1866mhz kit to 1600mhz or overclock the other one. If you try to run RAM on different clock speeds it will either not work at all or cause some really weird interactions.
EDIT: Also 16gb should do more than fine. I have 16gb and even 3D rendering (C4D) doesn't go past 14gb ram, atleast in my use.