In the same forest evolve caterpie into a metapod and don't evolve him again until after you leave the forest, squirtle is unskipable and charmander is unmissable but still skip able, but bulbasaur is hidden in a long stretch of grass on your way down from cerulean.( if you see charmander and don't have bulbasaur, GO BACK) evolve charmander during the fight with the magician that hypnotises you and evolve chareleon during the arodactile fight with the mining event ( right after the jiggleypuff event). In the veridian forest the Brock battle, catch caterpie, and MAKE SURE TO CATCH PIGIOTO when you find him he is a one time only catch.
Here are some tips: never evolve pikachu, bulbasaur, or squirtle.