I'm having trouble with the asset packs I downloaded (Mini first person controller and the Hospital Horror Pack) that were mentioned in the video tutorial, would it be alright to make my own character controller and level design assets?

And I just need to have the Flashlight that was taught in the tutorial implemented in the final output?

Of course!  The assets used in the video are only for example.  You can use them if you want, or not if you don't.