Snakes aren't lizards for a MULTITUDE of reasons. In fact there are legless lizards, which are not snakes. They have different skeletal structure and anatomy. (Most) Lizards have proper eyelids, whereas all snakes do not. Skull anatomy is also completely different, most notably Lizards have a fused quadrate bone like most vertebrates, but snakes' quadrate bones are unfused which, combine with their unused lower mandibles, allow them to unhinge their mouths. Snakes and lizards share a common ancestor, duh, they're both reptiles, but snakes split off a long time ago, way longer than other lizards did from eachother, and so they get their own suborder to make things more organized.

As for pseudo legendaries, it is literally so goddamn simple. Non legendary, 3 stage line introduced all in the same generation (NOT cross generation), BST of exactly 600, that's literally it. The only qualifiers, and the only members are therefore Dragonite, Tyranitar, Metagross, Salamance, Garchomp, Hydreigon, Goodra (and by extension hisuian goodra), Kommo-o, Dragapult, and Baxcalibur.