The people at OWC do this sort of thing on a common basis -- test to see what combinations of memory will work.
If I've figured your mac correctly, this page: https://eshop.macsales.com/shop/memory/Apple_MacBook_MacBook_Pro/Upgrade/DDR3_White
says that the most you can have is 2 8GB sticks.
Haven't done it, so take this with a grain of salt, but most of the time using a faster stick is tolerated. It won't run at the higher speed.
There are few cases where some odd combinations of memory work. One instance where a mac could use a 4GB chip in the second slot, but only a 2 in the first. I've also heard of combinations that had flakey performance with certain size chips -- would boot, but crashed at random intervals. I suspect that the chips drew more power than the main board could easily supply, and so were running at a marginal voltage.
More ram isn't a panacea. Fetching data from ram into cache still takes hundreds of CPU cycles. At one point I analyzed a raft of test results spanning several years from tests on Tom's Hardware Site. The major determinate of bench mark tests was front side bus speed -- usually a divisor of the clock speed. This sometimes meant that under clocking the CPU could allow a lower divisor and a higher FSB.
Some apps are greedy. Chrome on my mac will just use whatever its available. As I speak, Chrome is 1 G, and has 14 helpers ranging form 2 GB down to 400 MB.
Certain apps can really use more ram. Photoshop. Final Cut Pro.