should i use my SSD for system OR games?

If the ssd is large enough use it for both. Some games with long loading benefit hugely from being on an SSD, some you will notice no difference.

I would definitely have the OS on an SSD and most game platforms like steam have a way to move games between drives so you can move the current game you are playing to ssd while still having other games on HD.

If you really CANNOT fit your games and the system files on the same SSD, then put the system files there INCLUDING your virtual memory file.  EVERY game will use swap space, and the faster it is, the better everything runs.

Yes you could. I do. I only have one 1 TB SSD. Works fine for me.

Like flash drives and memory cards, SSD drives eventually use up all available space when they write. That is why people continue using hard drives for their durability. SSD's have a finite life span. Think before you risk data loss on a faster yet limited life device. 

OK, SSD works just like RAM...except that the SSD does not lose information when it loses power.  

If you don't care much about reliability, then 128GB should be the smallest SSD you buy.  That will handle Windows just fine.  You can always use a (existing) spinning platter hard disk drive to store bulk files, like games.

Your SSD will be more reliable if you can bump up to 256GB or 512GB or even larger.  This is because the life of a SSD is limited to number of write cycles.  Smaller SSDs will use the limited write cycles faster.  So all else being equal, a 512GB SSD will likely last longer than a 256GB SSD...even if you store the same files on both. 

If you can afford a 1TB SSD, there is no reason not to install everything (windows, plus games, plus office applications, whatever...) all onto the SSD

Don't ever throw away a mechanical hard drive if it still works "OK".  You can always use the mechanical hard drive to back up the SSD.

Primarily, I would use the SSD as your OS drive.  Depending on the size of it you will have some room left to install one or more games.  Games don't necessarily have to reside on an SSD anyway.

the SSD needs to handle the operating system.  if you can afford it, get one large enough for your best [most important to you] game as well.  -- grampa

Depends on what size SSD you're buying, the Windows/Operating System will use about 50GB.

Would be best to install Windows 10 on the SSD, will make all software much faster and loading times.