It will be fine. Do it. They will run at the speed at whichever one is slower, but speed of ram has virtually no effect on the speed of a system. Toms has some articles about it.
Depending on motherboard, you may need to put the slower ram in the 1st slots (a or 1?). May need to set the timings manually (leave everything auto that you don't know about) Also, it may require you to change the command rate to 2T in the BIOS.
Will it probably work? Sure. Is there any reason to mess with it since your system will likely never use more than 8GB of ram and the rest will just be unused? No