can you mix ram ?

Solution
It will work fine. As long as the memory is the same DDR generation it is fine, and if the two speeds are different the faster ram will just underclock to match the speed of the slower memory.
This can depend on the board and memory combination.

In general it works most of the time, but from time to time, it will cause issues(no post,stability,minor errors).

If you have 6 gigabytes, you already have 3 sticks so depending on the board you may not have 2 slots free.
 

PatrioticPickle

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Should be fine, I have 2x 8gb Geil sticks and 2x 4gb G.skill sticks. But I have found it depends on your OS a bit too: When I was running windows 7 it would work fine for the most part. Then I had the occasional blue screen, reseating the RAM seemed to fix it for quite a while (this would happen maybe once or twice a month so not that often). But since moving to Windows 10 I haven't had any issues what so ever, no blue screens or anything.