There is a vast difference between heeling and loose leash walking. Heeling is a very formal, focused, intense behavior that requires a lot of concentration on the dog's part. Loose leash walking is for casual locomotion from point a to point B, and IMO, as long as the dog isn't trying to rip your arm out of it's socket, they don't need to be in any particular position.
When you say "guard dog training" are you talking about property guardian, personal protection dog, or bite sports? Each one of those has different requirements, and aren't exactly interchangeable. Since Dobes are "Velcro dogs", and prefer to be with their people, they tend to do poorly as property guardians (other than being that "scary looking dog down the street"). A personal protection dog and a sport dog, while they do undergo somewhat similar training, are not the same. And finding a good trainer for a PP dogs is difficult, and can be extremely expensive.