Can a Galaxy-class's saucer section join onto a Nebula-class vessel?
I can’t find the answer anywhere but they both seem identical.
In theory, one could probably be made to fit the other. The saucers are of a very similar shape and size. In practice, however, the way that they're joined is very different. The link between the saucer on the Nebula class is almost 50 metres longer.
Image Courtesy of John Eaves
As noted by Rick Sternbach, the similarity didn't go unnoticed by the producers.
The Nebula is sometimes thought to be smaller than the Galaxy class. At least this was the original intention by the designers. Rick Sternbach writes at TrekBBS: "The Nebula class was supposed to be smaller than the Galaxy class, IIRC; the saucer shape is similar, but the bridge module is larger in proportion to the saucer size, as are the windows. The fact that the Nebula saucer looks like a Galaxy saucer (shape, colors, various design elements) doesn't exactly help the situation, but there it is."