My trainer picked my Cavalier from a breeder who has been working for 6 gen to improve the patellas and hearts of the breed (less focus on CM/SM, sadly, which leaves me unbelievably paranoid). It has resulted in a considerably larger than breed standard cav, but with healthier hearts and knees. As far as I know there was no hearing issues in this line, but she is learning hand signals/basic ASL along with verbal for all commands, and the verbal will eventually be faded.
I am not very active due to my disability, so it will be a lot of in home work, suitable for a small breed. Sadly, medical conditions accompany any pure bred, but when working with a SD it is good to know the line before selecting, so "hybrid vigor" from a shelter dog can often be too much of a gamble for working as you do not know the heritage of health problems, etc.
Mind you, if I were requiring a dog for mobility, I would need something much larger! (and probably find myself on a forum warning about hip and disc issues before the age of 1 >< )