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Having the stronger or weaker starter choice doesn't make the rival any more or less difficult outside of the first encounter with them at best.
Reason being is that you don't fight your rival "starter vs starter". At any point you can have Pokemon not weak to their choice, or outright stronger. There's no reason for you to say "Wow, how can my Grass starter possibly beat their Fire starter?!", when the reality is that you don't need to use your starter in the first place. Just beat that Litten with a Slowpoke.
A rival should be a challenge outside of the starter they choose. The starter for both the player and rival is literally irrelevant in determining any sort of game difficulty, truly.
Edit just to add: And the fact that an NPC's team can't change to the extent of the player's means no matter what you pick, you'll always know at least that one Pokemon that will be on their team. You can always prepare to counter it. A rival using the Fire starter will always continue using the Fire starter, and therefore always have the weaknesses that come with that. So again, as soon as you can start altering your team, the rival's choice means absolutely nothing.