this is my answer from lots of other threads (just copied them):
this has been asked a lot by others
answer is
Yes, you can mix every RAM you have
at a chance of it can fail (most likely around 10-20%)
if the mixing success,
the RAM speed will matched to the slowest one (i.e your 2400Mhz RAM will match its speed to the 2133Mhz RAM), and the memory bandwidth will accumulated ( 4+8 = 12)
if it fails, various error happens (collection of other's experience of fails)
If you want to attain 100% success chance, then your option is to go for a dual ram kit (this RAM kit has been tested to works as Dual-Channel)