1) I would try it myself. But you don't want to run out of space on the system drive. Have you made sure that things you don't need, like the hiberfile (if you don't hibernate) are turned off?
2) Different people will give you different answers. Some say that the manufacturer already overprovisions consumer drives, so you don't have to. I've seen people write that you should stop at 50 percent. 25 percent free seems to be around the average of posts that I have seen.
3) Depends on your money situation. If it's not a strain, get a bigger one. Either clone your OS to it or keep your OS where it is and add the new SSD, then install games to it.
4) Old SSD = fast. New SSD = fast. Unless you do massive transcoding or benchmarking, they will both seem fast. Don't worry about it unless your benchies are important to you.
5) Use it as storage or, as I said above, keep it as OS drive. If you can't figure out what to do with it, send it to me. I will definitely find a use for it.