Hi Rosanne
Welcome to the forum. All Traditional and regular oils have different drying times, but usually are not a problem if painted all in one setting (alla prima). Are the cracks in areas where the paint was thick?
I have had one painting crack, but it was where I had applied the paint much thicker than any place else. I had also used a small amount of fast drying medium to it since it was so thick, but that was probably the mistake I made.
I don't think you would want to use fast drying medium in some paints and not others if you are painting wet on wet. I would think that it would increase the amount of cracking. I don't usually paint alla prima, but perhaps others in the forum have more experience at that than I and can answer you better.
I will be moving this discussion to the "Painting Discussion" forum under "Water Soluble Oils" so it will be more easily found from searching.