I don't see any problem with the sketches and those magnetic holders. But in case of oil cards, you might need to seal the surface with a barnish to avoid the contact with the air, because this paint with the time hardens and it is possible to break. And because it takes so much time in drying (depends on factors as the layers of paint, materials, etc.) it's recommended to do it one year after finished the painting. But probabbly you don't find enough cards like these.
Acrylics can be barnished too, but is not necessary cause that paint dryes very fast and it's plastic. So don't need any other care.