It may also well be that the "generic" Galaxy saucer is fully compatible with the Nebula, but is only installed in exceptional cases, because it's far from optimal for the application. Say, during the Dominion War, standardized mass production of Galaxy saucers might be necessary, but the Nebula class would have no use for their impulse engines for reason X. For this reason, even the mass-produced saucers would have their engines faired over, in the manner we see e.g. on USS Bonchune - unless they received truly generic saucers that would soon be transferred to Galaxies and for that reason would retain the dead weight of the impulse engines.
A dedicated peacetime Nebula would have the completely different impulse engine arrangement familiar from the earlier physical models, such as Phoenix or Farragut. Transferring such a saucer to a Galaxy would be possible, but then the saucer could not perform detached operations, and would be reduced to mere emergency separations.
Perhaps Starfleet in the Dominion War wanted the detached operations capabilities of the Galaxy class, even though we virtually never saw this in combat or demonstrating combat value? Hence the "standardization" on Bonchune style rather than Farragut style saucers for the Nebula class, too. It may or may not have given the class the detached operations capability, but it would have kept the dockyards in the business of providing such capability for the other half of the fleet.
Timo Saloniemi
