While to me GSDs are the only breed, and all the rest are just dogs, I disagree with whoever gave you that assessment.
You really should love dogs, or most dogs, different shapes, different sizes, different purposes. When someone tells you they know how to manage big dogs, they have had plenty of labs, well, it is nice to know some of the different traits and characteristics common in gun dogs, and how that might be different for people getting a herding dog for the first time.
Knowing dogs in and out is what breeders really should be thriving for. If someone says they have trained border collies and goldens, how is training a shepherd different, well, actually I do not think that people really ask that question, but you can probably ascertain that if someone is accustomed to how border collies are, they will probably do pretty good with an active GSD. Probably.
But breeding several types of dogs at once, yes even three, that is harder to stomach. If you start out with sheps, and learn, and continuously learn bloodlines, and dogs and dogs and more dogs. If you spend time in the various type of training venues, clubs, shows, eat, sleep, and breath sheps, until you are an expert in many of the different aspects, and you want to add another breed of dog, that would be the way to go about it. But you would then have to keep up with two breeds. Two sets of bloodlines, watching various dogs of two different lines, what they produce, where they are weak, where they are strong and the whole nine yards.
I have a heck of a time learning all I need to know just about shepherds. The more I learn, the more I realize there is so much more to learn. I know that I could not manage more than one breed. But that does not mean that someone else does not have the capacity to do so and to do it well.
You would have to be able to look at yourself and measure yourself with a critical eye to know whether you are keeping up well with each of your breeds.
Health concerns, bloodlines, various training venues, training menthods, different lines (working/show, field/show) different types if separated into color, and all the rest.
It is not something where you can read Von Stephanitz's book and be an expert. It is a living base of knowledge that must be constantly followed.
Do not believe everything people say. Many people apply a thought they have for themselves to everyone in general: i.e. I do not like prong collars, I will not use them, nobody should use them.
Heidi Ho, Odie
Joy-Joy, Bear Cub, Hepsi-Pepsi
Cujo2, Karma Chameleon
Ramona the Pest, Kojak -- who loves you baby?
Tiny Tinnie, Susie's Uzzi, Kaiah -- The Baby Monster.