Aht
That's a very popular pokémon fic on space forums that uses mew as one of the main battle pokémon. Mew likes to have fun so he sneaks away and transforms into different pokémon for the mc to catch, the mc will use that pokémon till mew gets bored or gets found out and he goes and repeats again as a different pokémon. Just giving you ideas.
Really his second Pokémon is mew? A legendary for his second Pokémon so more plot armour..not to mention vulpix in its baby stages can make articuno fight to which articuno wasn't able to one shot? Sorry got to drop this it's so farfetched. The MC will literally just need to call mew anytime he struggles not to mention the fact you've listed like 4 people who have legendarys as if it's normal.