Like the old forza games where they dident make a designs when you painted a car ?
i dont paint so i’m not sure but you can do it with tunes. you apply/load it to the car without saving it. it stays on the car without taking up a save lot. try getting the biggest vehicle you can and applying the decale to it without saving. in the bottom right of the screen with tunes it shows you what button to use for apply vs save. if you can just scale it down as much as you can to apply more decales to the same car. if it works it might be worth getting that delivery van and peppering it with decales just to save them without taking up slots.
No it always takes up a slot, there is no way to just save it on the car. It’s really annoying because as well as painting proper designs I like to keep some of my cars stock with just a personalised number/ licence plate, in 4 I could do that and just save it to the car, now it takes up a catalogue slot 