The quick, dirty answer is yes, your motherboard supports RAM sticks of different sizes. But there's more to it than just "will it work?":
Your motherboard supports "dual-channel" RAM: that means that, if you use 2 identical RAM sticks, it will be able to double the rate at which it reads data from them. So with 2 equal sticks, you double the speed.
If you use different sticks (different capacity, or different timings), they will both work, but it will not be in a "dual-channel" configuration, meaning that they won't be able to work at twice the speed.
How does this affect you? It depends on what you use your PC for. For every task that is memory intensive, if you have mismatched RAM sticks you will be enduring a performance penalty. If you just game and consume entertainement, this will probably be fine, you would be slightly penalized, but not a huge deal.
Also, if you mix RAM with different speeds, it will work at the slowest rate.
The decision is yours, of course. I personally wouldn't bother buying another stick of RAM. 8 GB is plenty of RAM, and unless you keep tens of tabs open on your browser while gaming, you probably won't be filling it. I have had 12 GB of RAM since 2013, and never, not once, have i seen more than 8 GB in use, even with a couple of VM opened.
Hope it helps!