Any scale (or part of a scale) can be used at any time. It depends on the song. All we can do is give you options, but you're ear is the ultimate judge of what's "right" or "wrong" to play. If it sounds pleasing to your ears, use it. If it doesn't, then don't use it. I mostly use the major scale for all my little melody runs. I also use the pentatonic and blues scales during shouting music.
The chromatic scale is basically playing every note of the piano. This scale is used in every other musical scale because it contains all notes.
No question is a stupid question, especially when it comes to music. Hope this helps!
