As I have confused many with this, I will now explain that "grandson" has nothing to do with gender in this context, no more than it makes any sense for George to be Nanase's "Gramps". That's simply the role of the rival character that Nanase has fallen into in this parody.

That, and I'm tickled by subverting gender expectations. Nanase-Bubbles is still she/her regardless of being George's grandson.

Types

I'm fairly certain Fire starters are the most popular in Pokemon, though it will naturally vary by generation. The most recent one had a grass-flying owl named Rowlet, for example, and that seemed the popular choice. To be fair, however, Rowlet was cheating by virtue of being an owl.

Speaking of types, one can argue that professor Grandpa-whoever was giving his grandson the advantage by letting them choose second, meaning they'd be able to counter whatever the player chooses.

While this is true, elemental attacks weren't available right away in the first game, meaning this advantage meant nothing in the first fight, and then you go and start building a whole new team. It can be argued this "advantage" means very little beyond establishing the min/max nature of your rival.

That, and Professor Oak's philosophy, as I understand it, is sort of anti-min/max (meaning he's about love and friendship and all that, and not having "the objectively best" team). It doesn't seem in-character for him to try to give his grandson a monster based on what would best counter their rival.

As such, I continue to find it odd that you, the not-grandson, got to choose first. Maybe the professor wanted to see what his grandson would do in that situation, suspecting how his grandson would go about things, but it seems to me like the easiest way to put him on the "right" path would have been to not put him in that position in the first place. To instead have the first monster be something he chose without such calculation.

Instead, your rival is made to go second by his own grandfather, is somewhat understandably resentful about it, and goes right for whatever will best counter what the new kid got to choose first.

I stand by "bad grandpa-ing".