You are certainly right to start on a test area with the least abbrassive combo first and then work up till you find one that is removing the swilrs to your satisfaction... Once you have done that, then I would use a finer polish (if you have used a higher abrasive polish) to refine the finish - the reason we do this is aggressive cutting polishes can leave there own marring on the paint finish by the nature of the way they work, so following with a finer polish will remove this light marring and give a finish thats crystal clear ready for your glaze/sealent/wax.
Yes - even on the test area, if you've gone up to an aggressive polish, follow up again with the milder abrasive even though you have already used it as the use of a more aggressive combo may have inflicted its own light marring that you will want to remove.
Re: the PC use, it can be quite daunting at first, espeically with the noise and the vibration, but dont worry - its a very safe machine and unlikely to do any damage to your paintwork... feel confident in using it, relax (dont tense up) as this makes moving the machine and keeping the pad flat on the surface easier, take your time and ultimately, enjoy the process of transforming the finish on your car.
