Has anyone ever seen a well trained/behaved breeding stallion?

Absolutely! The majority of the stallions in the cutting barn I worked at were very well behaved, and if you didn't know it was a stallion, you would think it was a gelding. Also, have a friend who's stud is as friendly and calm as any gelding and very well trained. It all depends on the training and mentality of the horse and how the owner handles them. All of these stallions I've mentioned have been bred.