RAM needs to be run at a common frequency and be of like technology. In other words, you don't run something like DDR2 or DDR3 system RAM together, and while required timings may be different between various memory sticks in the system, so long as timings are all kept in a range compatible to all of the sticks being used, you can go that route. Understand that if you are running one stick at speed X, expect all sticks in that bank to run at speed X.
There is no guarantee the BIOS will set the RAM timing to the lowest common denominator. It may take the first SPD it comes to and time things off of that. If planning to run mismatched RAM sticks, I strongly recommend reading up on how to time the important parts manually, or at least...