I believe you are correct that the dash in the impala from 94-96 is the same, it is not in the Roadmaster. They opened up the area for the head unit to accomodate a Double DIN instead of a 1.5 DIN opening in the 96 (possibly the 95).
According to the Crutchfield site, the Fleetwood is a Double DIN for 1994-1996, so the dash on the Fleetwood most likely did not change during that time, unlike the Roadmaster. The Caprice has a 1.5 DIN opening from 1994-1996, same as the Impala.
Again, this is all based off of what I am finding on the Crutchfield site, which tends to be pretty solid for vehicle information. It is the little subtle differenced between the various BBodies that are strange. Why they would change the stereo opening for one or two years on the RM but not on the Chevys, but the Caddi has a Double DIN all 3 of those years.