I am not familiar with the class. But if skirts are not allowed what is done with cars that came from the factory with skirts, like Thunderbirds?
I think here you are talking about fender skirts, not under body skirts which is what rule 4.CC.7 is referring to. However, generically, rule 4.CC says any device that controls or limits airflow in or around the car and is not part of the original
body (emphasis mine) is considered streamlining. A fender skirt is not part of the "body". I can't find a specific reference to fender skirts but have the sense from the past that they are not allowed. May be wrong there. Looking at photos of many cars in many classes I do not see ANY that appear added. If it was helpful (and should be) and was legal I'd expect to see some. An exception was a Honda Insight which did have in the photo I saw. Don't know if ran that way. Moon discs are legal. I have seen reference to wind tunnel data suggesting they are of minimal help. Look nice though.
However, watch wheel offset so as with modern street cars the wheel/tire is not outside the body. A factory skirt as on a T-bird, may pass some places but not sure legal SCTA/ECTA as a bottom line. I suspect if you show up with them, no one will say boo but then again, been wrong before (well, once or twice--:) )