Mew is a Gen I Pokemon. Arceus is a Gen IV Pokemon. Case closed
R/B/Y only had the original 150 because no Pokémon from other regions had migrated there. As better communications were established between the regions, travelling between regions became more popular. Pokémon were migrated to regions that they were not originally indigenous to. The Pokédex was expanded to incorporate new Pokémon as the Main Character discovered them. As the character had never gone outside of Kanto, no Pokémon from other regions were included.
As for the question of which came first:
Mew's Pokédex entries say that it is the ancestor of Pokémon.
Arceus' Pokédex entries say that it created the universe before anything existed.
I think that it's fairly obvious which came first...