The ground rod is only required to be #6; regardless of the service size. The MBJ on the water pipe comes from 250.66 which is based on the size of the service entrance conductors. But the real question is how do you get 1200 amps thru a 600mcm without burning it up? Should be 3 600s in parallel (which will change your MBJ size).
I've run across that setup around here. Older wood frame apts, no bldg steel. They used to treat the long buildings as two separate and feed both ends. No fire walls; only reason we knew they were separate was to see the service drops on both ends and check out the electric rooms. They stopped doing this quite awhile back. Since these buildings are older, they almost always had copper (or steel) water pipe, plastic wasn't in use yet. We bonded to that.
