This is factually incorrect - Please read the books. There are whole sections about the Dragons becoming less strong. We have no idea where on the power scale Dany's dragons fit other than knowing they are way way smaller than Aegons.
Um, no. It was never suggested that the dying of the pre-Dany dragons would have any bearing on how big hers would get. Jorah straight up says that given food and freedom that they never stop growing, so they could very well approach the size of Aegon's three. Tyrion (and probably one or two others I can't recall) talk about their scales as well.
It's heavily implied that they're damned near invincible when healthy and grown.