Lady, EVOLVED 
can you battle using Johto pokémon in the Kanto gym challenge
i would lean “no”, evidenced by the fact that nobody in Kanto seems to use them (battles against champions notwithstanding), except that there are some “Johto” pokémon, like Slugma, which are native to Kanto but not in Oak’s original pokédex
either Oak left out some Kanto pokémon to keep the Kanto dex to a round 150, or there are some Kanto pokémon which aren’t considered legitimate for training and battling, or several pokémon were introduced to Kanto over the course of like three years??
why would Moltres be ok but not Magcargo??
so now i’m leaning towards Oak limiting the pokédex to 150 for the sake of keeping it a round number, and the actual regional dex for Kanto and Johto looking more like the one in HeartGold and SoulSilver, with the possible addition of newer evolutions like Sylveon and Dudunsparce
but that then introduces the question of why nobody in Kanto ever uses a Houndoom
@Lady whoa i forgot houndour is a kanto-only mon from gen 2.
@alyssa yeah it’s nighttime only which does explain to an extent why it isn’t in gen i, but also, clearly it exists in kanto,
i would believe that the Indigo League kept the number of usable pokémon small for Reasons, except that the same league is in charge of Johto (and was fine with a list exceeding 250 there) and they clearly had no problems including, you know, Zapdos