id say over 95% of the time unmatching RAMs will work together. however, the other day i paired 2x2 1333 mhz G.skill with my current 2x4 1600 mhz G.Skill Aries RAM and i started running super choppy. I knew something was wrong right away when my boots time was almost quadrupled. Idk why this happened, but it was very obvious.
if you install and everything seems fine from boot i would recomend doing some simple stress tests on you own (without downloading software). a good way to do this is think of times you are able to produce system lag with your current set up (ie, heavy gaming, multi tasking, video editing, etc.) put this same demand/workload on your new setup and determine if its faster or slower for yourself...