When singing a musical theatre song, but not something like Phantom Of The Opera more like Frozen or a contemporary song. Would you still want to use that very Tall shape in the mouth? Or would it have to change?

Depends on the pitch and where it sits within your range! Tall position (for higher belting notes) encourages more mixed coordination so we don't drag chest voice and get "shouty". But you can use a neutral/wider position for lower pitches if you like that tone. 

I'd encourage you to watch live performances of professional musical theater singers, and see what they do! You'll probably notice a variety.