Looking at some pics of the u.s.s. Pheonix, as I believe it was the first nebula class ship seen on screen. I can't see the rear of the saucer: http://en.memory-alpha.org/wiki/USS_Phoenix
So I'm not sure of the shuttlebay, also, I noticed that there are no impulse engines on the saucer, The farragut: http://en.memory-alpha.org/wiki/USS_...ut_(NCC-60597)
) appears to have a galaxy style main shuttlebay. but no saucer impulse engines. this would suggest that it cannot seperate. also pics of the uss bonchune http://en.memory-alpha.org/wiki/USS_Bonchune
has no saucer impulse engines, but has more numerous windows, similar in arrangment to a galaxy class ship. maybe the galaxy class ship was built as a "fly the flag, rally the people" type ship?