So these skins, do they apply to ALL cars in the race of the same type or are they specific to a particular car number (which can be unique?)?
Only to that specific number, if I understand your question right.
Each car has its own directory in the cars/f2000/skins location.
The directory has the skin itself (.tga), the menu icon (.bmp) and the cockpit file (cant remember the extension of this because I never edit it).
So if you copy an existing directory and change the name to eg shark#124 then this makes a car number 124. Simply replace the .tga file with your own skin and that's it.
Very easy compared to gtr, there's no need to unpack and edit anything.
That skin will only be for that car number so even if there are 15 F2000s racing only one (although there can be more if more peeps choose the same skin) will be in your colours.
The car number is just a graphical thing afaik - the actual racing number is part of the player profile, as is the helmet.
Hope this answers?