Hello everyone! I’m really enjoying playing Pokémon Light Platinum. While I was playing I’ve been wondering if a speedrun of this game (or other rom hacks) is allowed?

You can speedrun anything you want; whether the game will be accepted on this site or not is another matter. There are plenty of SM64 romhacks on the site so it's not out of the question.

Feel free to check out https://speedrun.com/pkmnfangames 
Lots of romhacks and fangames to choose from!

You can also join the discord here: https://discord.gg/vFYQyzF