Exactly, this is MLP not other stories. The rules in those stories do not apply to MLP, so we use the rules we see in the show and in the show we have not seen or heard about any half dragons so far. If we see or hear about half dragons in the show then it becomes a possibility, but until then it is best to assume that it is not.
You are the one who said that anyone could own a store, I was just pointing out that is not true in real life so it may not be true in MLP.
Twilight hatched Spike and then she raised him, that says mom to me. Also I just think their relationship is more interesting as a mother and son one rather then just best friends or even as siblings.