Mixing RAM types of different CAS latency is not recommended. If it does work, it puts an extreme burden on the memory controller which will struggle to maintain stability.
If the memory is free, you can try it, but the chances of it working and being stable are very low. Might not even BOOT. If it does, day to day computing tasks could work, but the moment you challenge the system, it might lockup, BSOD or fail. I wouldn't bother trying, as using matched kits or at worst using different brands together with identical latency would be a better choice. Keep in mind, even (same brand) unmatched kit can be iffy.
The days of pairing faster RAM with slower RAM and running at a slower overall speed are over. The "tolerances" have been tightened due to users performance and over clocking demands. BIOS and XMP profiles are more stringent as well. My advice is don't do it. You are risking system stability.
