Slip Mat Thickness, Tracking and Dampening

Hello,

I recently bought a Pro-Ject Carbon Debut turntable and I replaced the standard felt slip mat with a slightly thinker one. Does this have any affect on the tracking of the stylus in the grooves? It seems to me that it is a bit more quick to skip when I walk by. The TT is on an Ikea Kallax shelving unit which is in turn placed on hardwood flooring, which I know isn't really an ideal setup to begin with.

Part two to my question: Where is the best place to dampen vibrations - between the floor and shelf or between the shelf and the TT?

Thanks!