So I recently learned what that format means, the slash basically being of (V of V), I always confused it with slash chords before.

What I am still a bit fuzzy on though is what scale are you pulling the secondary chord from?

I can understand that in the cases I posed in the title, it wouldn't really matter if it's major or minor as the fifth note is the same, and the capitalization would dictate the major or minor nature of the chord.

But lets say hypothetically, this will likely be a nonsense use case it's just for example, you were doing something like iii/V. The third chord starts on a different note depending on major vs minor

So lets just say in this case the V is C major. Couldn't that mean E minor or Eb minor depending on the scale used?

Is there some dictation rules regarding this?

My guess in this case is that it'd likely be referring to E minor from the major scale, bur hypothetically if you wanted to write the numeral notation for Ebm to C, what would that be?