For a hopolite with a focus on destroying magical effects (Unraveling the Fabric of X, and Disenchant) Hermetic or non hermetic spells as well as casting Perdo Vim spells to end effects caused by magical creatures but not faeries, demons or angels, would that be a minor or a major focus.
If it is a major focus, would including faerie, demon and divine effects still fit, or does this make it too broad to be a major focus.
I'd allow it, on both counts, as long as in both cases the Focus is Destroying or Disentangling magical (or supernatural) effects.
In the case of the minor focus, in meets the test of being a reasonably discreet subset of a single combination of Arts (PeVi); In the second case, I'd even allow the destruction of Supernatural Beings to fall within the Major Focus of Supernatural Oblivion. As a rule, you can collect every effect covered by a single pairing of arts within a Major Focus with comfort.
Certainly, its no broader as a focus than the Major Focus of Damage, nor is the minor focus broader than the Clan Sirnas Minor Focus or Wards.
Forseeing the obvious objection, you should remember that the Pralacian Minor Focus on Exotic Magic includes spells that deflect, mutate, change or even convert or boost non-hermetic magic, not simply spells that destroy it.