While leather is a fabric that is notorious for moulding to fit one’s feet, it is unlikely that significant shrinking will occur to the point where the shoe no longer fits. That is, unless it has been exposed to high heat and moisture. For example, if your leather shoes get soaked, and you try to speed up the drying process by applying heat, this can cause shrinkage and stiffness in the leather.
Treated leather is ideal for wet weather, as this ensures water resistance/waterproofing characteristics. Thick, full-grain leather is naturally more water-resistant; therefore, it is always recommended that you check the grain of your leather. Untreated leather might not fare too well in wet conditions, and is probably best kept for warmer weather.
Leather boots tend to have a good combination of breathability and protection. Since leather is a natural material, it comprises tiny pores that allow airflow. That being said, if leather is heavily treated, this can reduce its breathability.
It is often recommended to use a soft bristle brush to remove dirt or debris from suede. Tougher stains might require further effort, so always ensure that any products used specifically state that it is safe to use on suede.