the autoflower will go into flowering based on time, so it won't affect the other plants if they stay in the veg-cycle based on the amount of light-time it gets per day..
on the other hand if you flip to 12/12 to get the photosynthetic plants to flower, then it means the autoflower will only receive light for 12 hours, and will therefore not grow as much as if it had gotten 18 hours or more..