No. Only 18 inch tires fit an 18 inch rim.
Yes there was a factory option for 17 inch wheels
Yes.
yes
The 1995 Pontiac Firebird can be fitted with 18x8-inch wheels granted they are designed for this application. A tire size that will work on these wheels are 275/40/18.
Stock it comes with 18 inch on the GT
27
18 inch back not sure on front
Keep the diameter of the wheel/tire the same with the 17" wheels as you were running with the 18" setup. That will require that you install a set of tires on the 17" wheels that have a larger diameter than the tires now currently on the vehicle. w www.tirerack.com is a good place to find out exactly what size tire to buy.
The 18 inch wheels have very large surface area which reduces the pressure that the car exerts on the road.
Depends on what size wheels the frame is made for. You could have an 18" that is meant for 20" BMX wheels, or 26" MTB wheels or 29er MTB wheels. Getting the wheel size wrong is unlikely to result in a nicely rideable bike.
I have seen a few on the net with CLK 18" alloys on, so my guess is quite possibly