Regarding the original question, I have a question back: How did you price your Catch (particularly the access portion of it)? After all, that tells you how rarely to expect to see an attack that qualifies.
If you gave it the +0 option (only a couple of people in the universe have access), then it seems reasonable that only a very restrictive definition applies. In the case of "Outsider" as a Catch, probably only the attacks of an honest-to-God Outsider (ex: He Who Walks Behind) would count. For story purposes, you might also claim that a (exceedingly rare!) Item of Power -- crafted by an Outside, infused with its essence, and wielded by a loyal believer of that Outsider -- would count. On the other hand, if you gave it a +2 for accessibility (anyone could reasonably get access), then you need to lower the bar enough to make that statement accurate. In the case of an "Outsider" Catch, this probably means that anything touched by Outsider power would have to qualify. So, for example, the Outsider equivalent of "holy water", or any item "sanctified" in a ritual using Outsider magic. In between, +1 would probably cover attacks made using Outsider magic, or any attacks by a creature "of Outsider blood", including Scions.
By the way, the rarity of the creature plays a role, here, too. Fae are much more common, so the above changes a bit. The +0 option would probably have to be limited to attacks made by one of the Fae Queens themselves, for example.
At least, that's the way I would look at it.
Switching gears for a moment, where does this "Ghouls = Outsider Scion" stuff originate from? I can't recall anything that would indicate that they have Outsider blood or are otherwise related to Outsiders, but perhaps I'm missing a reference somewhere. I know there is the theory presented in OW that 'mundane' Ghouls are Scions of Uber-Ghouls, but beyond that I'm coming up blank. I almost got the impression from what sqlcowboy was saying that "Outsider" was being used as a stand-in for "supernatural" (otherwise I have no idea what "breeding true" has to do with it).