Not a good idea. But if you insist they you need to change the aspect ratio. For instance if you have a 215/60-15 tire and want to go to a 175 then you must go with a 175/75-15 to keep the overall diameter close to the same. Otherwise your speedometer will be way off and handling, performance, traction, and fuel mileage will be adversely effected. My advice is to stick with the OEM size.
Yes, that is a reasonable change.
That is not a tire size.
The size you have is about as cheap as you will get in a 16" tire.
If you are asking what is a good replacement for this size then a 175/60-15 will work just fine. A 215/50-15 will also work.
No, it will not.
215/70SR15
215/60R15
The tire size that the manufacturer recommends is 215/75/15, do not change tire sizes, unless suggested by a pro. By the way, look on the inside of your driver's side door jam on any vehicle, the factory recommended wheel and tire size is there.
215/55/17
215/60 R15
215/70SR15
215/75R15