They should need the same voltage. Look for the voltage of the RAM. The quantity of RAM has nothing to do with the performance.
RAM with different frequency and timing can work together. Your bios will choose the frequency/timing of the slower pair of RAM.
Even with the same brand, frequency, timing, sub-timing, the same chip and voltage, they is always a small possibility that they won't work together. It should work though as long as the motherboard manufacturer's page says both types of ram are compatible., but there is no warranty. The only way to know is to try them.
I talk about the compatibility of different pair of RAM, not between two different stick of RAM used as a pair.
Wait for the boxing days or black Friday. DCS need 16GB of RAM to work properly.