As far as the big about bridges and engine rooms, there may be minor differences but the layout would be the same. This is broadly supported by "The Ultimate Computer", "The Omega Glory", "The Tholian Web", and "Where Silence Has Lease", to name a few.
Insofaras the origin of this thread, I would think it would be possible to construct a mission-specific saucer for a Galaxy-class ship for military purposes (or a Sovereign, or a Nebula, or an Excelsior, for that matter) but I don't see it as practical. It would seem more
practical to build a squardon of Defiant-class escorts, load up the unmodified Galaxy with troops, and take care of business on the mission that way.