You are exactly right mutt dogs are not hybrids, they are all the same species whether St Bernard, or Spaniel.
I consider crossing different cichlid species and making them mutt/hybrids is even worse.
But in the end, the result is almost the same.
I'm also a bit of a purist when it comes to line-bred fish. When you see how fragile EBJDs are, it makes you wonder about how when you breed only for looks, what's being sacrificed within.
Again, hip displacement and the line bred maladies in dogs comes to my mind.