I asked my friend who plays 7 string in a locally popular metal band.
In standard tuning, practice your 6 string playing on strings 1-6 skipping string #7, the same way you play D major and D minor chords skipping string #6.
practice playing power chords on strings 7-6-5. He says that's what the 7th string is for, playing power chords on the low end that are so deep you feel it down to the core of your body.
he also suggested 7 string drop-A tuning, you tune the 7th string down from B' to A', being in the CBG genre, that gives you A'EA on the low 3 strings just like a CBG. In effect you play it like it is a 2 neck git, strings 7-6-5 as an A'EA (1-5-8) baritone CBG and 6-5-4-3-2-1 as a standard guitar.
What scale is it? You could do bass on the low strings and guitar on the others like JL said. Lots of possibilities.