Kevin had some great points. Stallions are still horses, HOWEVER they do require more skill to handle, one of the reasons why NO clinician really wants them in their clinics.
One of the interesting things that Karen Rohlf says in her book is that in NH, it really focuses on the mental and emotional aspects of the horse, and some of the physical. In Dressage it really focuses on the physical development of the horse, and some of the mental and emotional....Karen goes on to say that in her teachings of Dressage, Naturally her goal is to bridge that gap where NH "ends" and Dressage begins. It's SO important to have the horse calm, relaxed, willing, attentive and keen, etc. BEFORE ever starting in the endeavor of Dressage. I've already started incorporating some of her teachings, on the ground and riding, and have had much success with it so far.