Your understanding that all non-residents in NB require a guide is correct, and the $600 fee does not include guides. I say "guides" because if all four rods are non-residents, two guides will be needed -- a maximum of three "sports" per guide. The trick is to find one resident with whom to fish.
While I have never seen a time after July 1 when that pool has not held fish, the fact that there are fish there in numbers does not mean any will rise to the fly. I have fished the pool many days in the past when no one caught a fish. It is, however, much better to know you are putting your fly over fish rather than just rocks.