Dunno what will happen with Slowpoke and Slowking, but there's at least Pokemons like Eevee, evolve to Espeon and Umbreon, Porygon evolve to Porygon2, Onix evolve into Steelix, Scyther evolve to Scizor, Chansey evolve to Blissey. That's all I know and then there's those lower forms, like Pichu, Magby, etc.
So I don't know is it worth to stack up since they can balance gen2 Pokemon stats as well.