>>28178254They add those 'mons for every. single. gym leader.
Yellow has a Mankey for Brock, they give you Pikachu and Paras for Misty, Diglett Cave is in Vermillion for Surge, Lavender precedes saffron (if the game worked as intended, at least)
Abra helps with morty's team and should be a Kadabra by then, Marill is in Dark Cave before Pryce, where Rock types cant deal with Swinub line. There was also Tyrogue in the cave, not to mention Magmar.
Clair is on a route with Skarmory (except in Gold) and you literally have to go to the Dragon Den to fight her.
I dont think there's any particular psychic or flying types for Chuck but at least Tentacruel resists Fighting. Oh, and it doubles as SE against Steelix and can more than tank a Magnemite's weak attacks.
Skipping ahead, they give you a Riolu in B2W2, when the first two gyms are Normal (weak to fighting) and Poison (evolves into a Steel type)