You can.
The consequences:
* the will work on the speed of the slowest module
* they might not work out of the box and you will have to play with bios settings to force them to work
* they might not work together at all.
Short answer: no
Long answer: yes, but you'll have to set it to the worst speed/timings of them, possibly relax it even more. In general - not recommended. Likehood of dual channel not working at all is there as well.
You can.
The consequences:
* the will work on the speed of the slowest module
* they might not work out of the box and you will have to play with bios settings to force them to work
* they might not work together at all.