I just started speedrunning and I was wondering if my run would be invalid if I used the Custom Knight mod. I wouldn't be using any other mods, and I would show the mods list at the start. Can anyone help?

Unfortunately not. Custom Knight requires installing Modding API, which is banned.

If you have more questions I would recommend joining the discord: https://discord.com/invite/3JtHPsBjHD

fwiw, just having the modding API installed is banned. so if you've previously played with custom knight you'll have to revert to a fully vanilla install.