Typically people go for 1 ssd + 1 hdd. Windows + Programs on the SSD, and files + games on the hdd. This does not impact the performance of the SSD. People have done smaller SSD + larger SSD as @Levent mentioned. You can also do two ssd's in RAID 0 (strip) to make one big drive, but then you risk losing files due to a single drive failure.
Yes, that is what I was getting at. In terms of monitor suggestions, BenQ or ASUS are pretty good in terms of quality for 144hz, but if you want to go cheaper Acer, Viewsonic, Monoprice, or HP are usually the trend I see.
if you're cpu is ancient then you're probably still running on DDR3 RAM. Most modern CPU's run on DDR4 now. When you upgrade your cpu, you're upgrading cpu+ram+motherboard
if you think you made a mistake, then check the pins on the cpu, see if they all seem relatively straight. But I've had this issue with one of my build, which resetting cmos fixed it
downclock speed, but also the timings. But overall not too experienced with this. Only done it once before with success
You see i'm not too sure. In theory if you manually adjusted the speed and timing it should work (not using XMP settings). Done it with two different G.Skill ram sticks