It is never wise to mix RAM. many differences between the sticks could cause issue. RAM with the same model number may not work together. I suggest replacing the RAM with larger units/matched pairs.
That being said, it will either work or be really unstable. you can try, install the RAM and then run the RAM through memtest86 for a pass or two. if memtest86 finishes okay you should be good. I would not trust the PC to do work until the RAM has tested good.
memtest is usually found on any boot-able linux distribution.