I do not believe that is a recommended practice. Sometimes your wheel might not be able to even accept a smaller tire. Best is to check with the tire store - they will know how much leeway you might have.
Can I use a smaller tire width instead of the original?
If by "width" you mean the width of the tread, you can usually go larger, but it's inadvisable to go smaller.
The width of the tread is an integral part of the design of the car and affects the load it can carry as well as the stopping characteristics of the vehicle. (The tread is "where the rubber meets the road.")
Cars are designed usually to operate with a narrow range of tread widths. A tire dealer can tell you the range of widths that are acceptable to install on your car.
Not unless you are changing the rims out as well. The rims are designed to hold particular width and if you go smaller they will nor seat properly and will blow.
And going smaller on the rims isn't a good idea either. Tire sizes are calculate to hold you vehicle to the road with maximum efficiency. Going smaller may cause you to lose control or put too much stress on the tire resulting in catastrophic failure.