That said, making negative Fate Points free compels seems a perfectly valid house rule, at least to me.
No doubt. I mean, sure the PCs start with fate points, er, fate
point. As in one, because they're not morons and spent all their refresh. It does seem somewhat unfair that someone who is fifteen points in the hole only acts a little more in tune with their nature than a free-willed PC.
I'd say that viable houserules could be:
1) Negative refresh NPCs gain fate points as normal, but cannot refuse compels until they have accumulated a number of fate points equal to their net negative refresh (this will require assigning refresh totals to the NPCs statted out in OW, since it only tells you how much they spent, not how much they overspent).
2) Negative refresh NPCs do not gain fate points at all until they've paid off their debt.
3) Negative refresh NPCs must pay more than one fate point to buy out of compels.
I'm leaning towards using 3) in my games to see how it goes. I'll assume everyone starts out as Submerged (so subtract 10 from their spent refresh). If they are between 0 and -5 they buy out of compels as normal. From between -6 and -10 it will cost them 2 fate points to buy out of a compel. From -11 to -15 it will cost 3, and creatures who are more than sixteen points in the hole can't buy out at all.
It is also possible that I will restrict the higher costs to compels against an NPCs high concept.