What I was told about single stage is that they put the "clear " into the color and when it cures the clear rises to the top layer so you have to be careful not to cut/buff/polish because it's thin enough that it's easy to cut right through. I have never tried to polish a single stage so I'm curious as well. One of my cars has a single stage and the surface is not as smooth as I would like. It's a good paint job so I didn't want to screw it up but it would be nice to be able to smooth out the finish a bit and maybe make it shine a little better in the meantime. The guy that painted the car said if I thought it looked ok to just leave it alone.
Depends what paint and how old the paint is if buffing will do anything. There are enamel single stage paint and there is urethane single stage paints. Old weasthered cheap enamel single stage paint won't buff out no matter what you do. Depending on the age and weathering on a urethane paint you may have a good shot of wet sanding with 2000-3000 grit paper to knock off all the roughness and then polishing. Depending on how good the paint is you could always wet sand and put a good urethane clear cost on top.