Its not recommended to mix them, which is why you should buy them in kits. But you might get away with it, I used to do it on a machine I had a few years back. It had 4 sticks of different sizes all by different manufactures, and it pretty much worked flawlessly. Some people try it and run into all sorts of issues.
Wont damage it to try. Any problems just go with your newest ram on its own.
When it comes to mixing ram kits they should have the same timings, CAS latency, frequency, voltage required and quantity of ram.
Even so, it may not work at all.
8GBs is quite enough for most user's needs, going above that aount of ram is required only when multitasking or heavy softwares.