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). RAM vendors often don't cover 2 different modules even of the exact same module, that's why they sell SETS of RAM.
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.
So IMO, you're always best just getting a matching pair, as it can be an expensive chore to buy and install mixed RAM, to find that they don't work together.