I'm going to suggest something, it may be a bit harsh but tell me what you think. I think you should state that "Your car should be in good working order, Break downs will not be stopped for and recommend UHF radios." At least the front and read cars should have a radio so they can communicate that everyone is accounted for.
I do think that 15 cars is a bit hard to work around, smaller groups may be easier. Maybe have Classic 1 and Classic 2 if there are so many cars. I like the idea of 10 cars. That last cruise we had was with 17 and it was hell. Maybe if it reaches over 12 cars split it into a group of 6 and one of 7??
Just an opinion.
JiP
Jon.