help writting metal songs.

I need help with witting a metal song with theory.

How do you know what chords to use besides 5ths?

Also what are good scales to write the rythem part with?

Also when you change a scale durring the solo, does that mean the rythem part of the song need to change to that same scale?

Thanks

About the scales, depends on your taste, I would go with harmonic minor or auxiliary diminished. There are some scales that sound good together with others , you gotta experiment and see what works for you, but in most of the cases you will have to change to the same scale.