I have changed the front brake pads on my 95 grand Prix with no problems. I went to tackle the rear break pads today and could not compress the piston on the brake caliper using a Clamp. So I pulled the whole caliper off and figured out that the piston just screws in beacause of the emergency brake. During the process of putting the piston back in, I did break the rubber boot off around the piston.
By not having this rubber boot on will it cause any problems? Should I buy a new one?

Doing the otherside was easy after knowing all i had to do was to screw the piston in to get it to go down, i used some vice grips.

You'll need to change that boot as the tear in it will allow dirt to get into the mechanism which could cause brake failure or lock up.

I agree with GPFred, you need to change that caliper before it seizes.