False.
First starter in gen 1 was either Squirtle or Bulbasaur. In gen 3 the Fire starter, gen 4 the Water starter, Gen 5 either Snivy or Tepig. Gen 6 and 7 were the Fire starters. In gen 8 I picked the Grass starter. I skip gen 2 as I only played that as VC, so idk if it counts (Chikorita in crystal VC).
And on any subsequent playthroughs, such as in the opposing game, I pick one of the others that I didn't pick first time round =3
Tpbm has tried a playthrough with only Pokemon of a single type.