hopefully you're running a 64bit OS and your motherboard and/or cpu can support 32G. check their specs, easy to find.
when you mix ram, you usually end up running at the slower condition - so mixing 1600 with 1866 means everything will run at 1600. with intel, that's perfectly fine anyways. with AMD, it's not ideal if you're using onboard graphics, but perfectly fine if you have a separate graphics card.