Dortokay

I just got a new Asus Vivobook laptop that comes by default with 8gb of soldered on memory and 4gb in a single replaceable slot, but supports up to 16gb total. I know memory mixing is a big no-no when it comes to regular slots, but will I run into any issues if I buy a regular 8gb stick at the same frequency off Amazon and swap it in?

My computer networking class at a specialty center I go to through my high school recently had a LoL tournament where at the end we gave away a gaming PC we built to people who bought raffle tickets to support the program. I ended up getting super lucky and won the rig. It has a 2080 and a 1900x in it. I'm obviously replacing my current 1060 with the 2080, but I'm not sure what cpu to use. My main rig has a 2700x, so which one would be better?