If you told me this eight years ago, it would have been enws to me.
Seriously though - it's not recommended. Any other pokémon you catch will seem to grow at snails' pace compared to your starters, unless you play through both games at the same time (which is no fun - repeating yourself is extremely tedious) and trade each pokémon you catch. Why would you want a Squirtle anyway?
Since all three starters have nice alternatives, there's no need to do this:
Ninetales is a decent alternative to Charizard.
Vileplume is a great alternative to Venusaur, and Victreebell is pretty good too.
Vaporeon and Poliwrath are both a lot better than Blastoise will ever hope to be.
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