I'm against adding extra rules for background cars as I find the current way of dealing with them works quite well. The only criteria should be if the picture is clear enough for the vehicle to be identified.

'Special', 'unusual' or 'interesting' are largely irrelevant terms I think because they are so objective and depend on who is making the judgement.
My attitude might be different to a lot of peoples but whereas I do have some preferred vehicles, I find almost all vehicles interesting. From cranes to tanks, cars to trucks, and bikes to steam engines they are all of interest. The age of vehicle (or of the movie itself) also makes for an interesting record of motoring history or styling. Even recent models within a few years or decades become 'rare' with the once common models not being preserved in favour or the 'interesting' ones. So for me the more that can be listed the better. Just make them as clear as possible (not blurred) and keep the number of background vehicles within an acceptable number consistent with the amount of vehicles seen in the movie.

Quantity is important and where it would be possible to list a great number of pictures some selection is necessary to reduce the number to an acceptable amount. However in those cases the ones that are most clearly seen for the longest period of time should be chosen I think.

I think the present system works quite well and applying a little common-sense is better than strict rules. I'm also fully in favour of other contributors making suggestios - either to add or remove cars - to keep things tidy.