So I'm using RPG Maker VX Ace to make my game (it's my first ever game btw) and I just learned that if I'm using default assets, RPG Maker requires the player to download a Run-Time Package. Is that fine in this jam or not?

It should be fine, there's no rule against it. Congrats on making your first game, btw :)

That's perfectly fine. Any engine can be used when making your game. 

Thanks!