you can tape the edge of the panel you are polishing, what i prefer to do is tape the edge of the panel that you are not polishing, so if you were polishing the bonnet, tape the tops of the wings.

ive never had a problem polishing edges of panels, but take care, and polish away from the edge rather than towards it!

colour doesnt really make much difference to me, the idea of polishing is to remove any defects, so swirls, etching, marring, etc and leave a perfect mirror finish,

this can only be achieved with slow careful polishing, if you rush it, you will fuck it!

take your time, and make sure you work the polish well if you are using megs or menzerna!

i wouldnt use g10, because afaik it doesnt break down, so you can polish and polish and just keep removing paint, what i would use is something like meguiars #80 or #83 or the menzerna range of polishes, which breaks down as you work it, so refining the finish as you go!

the vents are probably too small to polsih with the rotary, unless you have a small polishing head for it, i would use some fine polish by hand on those...