Diamond Delta, what you are describing is pretty much what happened with my local - right down to the NPC sorority in question.
I was part of a local sorority that was considerably smaller than the NPC chapters on campus (10 sisters vs. 90-100 sisters in each of the four NPC groups; total was 80). We liked being smaller - we were shooting for 40-60 active sisters. When we started talking about NPC affiliation, one of our concerns was that we'd be getting nastygrams from nationals about not meeting quota/total, but we were very up front with AEPhi about our intentions, and they were ok with it (not thrilled, but ok).
Looking back, I think we were a little
too small compared to the other NPC groups, but in a situation like this, where the local is more established and isn't too much smaller than the NPC groups, it could work beautifully.
(But I'm biased...
)