8 GB is typically fine for gaming, although more are starting to recommend 16 as some gobble up 6 gigs or so when running, leaving little for background stuff and the OS.
If you're a heavy multitasker, stick with 16. I sometimes run my 8 GB laptop close to running out of RAM just browsing and multitasking.
Compatibility issues between different RAM kits may sometimes happen, but it's never happened to me in 10+ years of building computers.