RBY Rumors and Fake Cheats

[Poryhedron]

The most convincing Mew legend I remember hearing is that if you caught every pokemon and talked to the man who said to talk to him again when you finished you pokedex, he would give you one. I suspect this is false, but I am unsure. Anybody know?

Having accomplished that very feat more than once, I can say with confidence that it's false. The man just gives you a diploma. There was a rumor that he gave people a Mew in the original Japanese version, but I've heard that this is false as well.

On a slightly-related note, I want to find Missingno once again on my Blue version, and I vividly remember seeing it many a time in days of yore, but now I can’t seem to find it. Can anyone give me some clues or pointers?

There are various ways to encounter Missingno., but perhaps the simplest and quickest is to talk to the man in Viridian City who shows you how to catch pokémon, watch him catch a Weedle, and then Fly to Cinnabar Island and Surf up and down the island's east coast. In that area you'll encounter whatever pokémon were in the last wild area you were in, and watching the Weedle capture screws up that data and makes Missingno. appear.