I put a small dent over my driver side rear wheel on my 2008 Chevy Impala. I touched up any scratches but I can still see a small dent. It looks like if I could take off the covering off over the tire, I might be able to get to it and tap it out. What will I run into if I tried this. I saw this niffty tool on YouTube called the slide hammer that works from the outside of the dent, which look to good to be true. Any direction will be appreciated.

To use a slide hammer and depending on the model you will first need to either spot weld a stud to the body or drill a hole for the screw tip to thread into. One takes a professional spot elder and the other requires a welder to fill the hold afterwords. Take your choice.

I don't spot weld and I'm trying to go the most in-expensive route. Is there a type of glue that I can use on paint that will not damage the paint and can be used with the slide hammer?

Not that I have ever seen. There is an outfit around called Pop-A-Dent that uses some method that seems to work pretty well. As I remember they are fairly inexpensive. The biggest problem working with sheet metal is that when you dent it, it stretches. So unless you have a very small door ding it will likely need to be shrunk back to size before you can properly effect the repair. If you are not proficient in body work, I would suggest using some bondo to fix it.