To be honest it's almost complete pot luck with different memory modules. You're ALWAYS safer getting multiples of the same RAM module (ideally even in the same pack).
Adding in different types of RAM sticks is a 50/50 bet, sometimes they work, sometimes they don't. The closer the voltages, speeds, sizes are, the more likely they are to get on, but there's really no guarantee. So mixing RAM is never a recommended practice, but it CAN work. As long as you set it up to so that none of your modules end up down-clocking one another.
On top of that, you're potentially putting yourself in more difficultly by opting for 2 different speeds. Firstly your motherboard will need to be compatible to run the higher speed, and then you may still end...