Well of course you can mix match any DDR3 on the market. It may happen they're not compatible either together or the mobo doesn'T support both models. But it is very rare and you could always do a BIOS update to correct that if it's the case.
Anyways, mixing RAMs mean that the system will use the slower RAM speed speed and latencies as default for every RAM sticks. But it is not a problem at all, the important thing is to get more RAM for video editing^^
Also with 12GB of RAM you could use 4GB for system swap, and the remaining 8GB as regular RAM. It would speed up loads by a large margin you know. :-)
Actually mixing RAM can result in a lot of headaches. You can try it but there are no guarantees and often two different types of RAM won't play nice together. If you check any of the RAM support forums you'll see the problems with mixing RAM.
Since you are using a Dual Channel CPU/Mobo you'd be better off with 8 Gb. or 16 Gb. as 12 Gb. will revert to single channel operation, which will be slower than dual channel.