I just looked at the S550 manual online(you know, manuals are available on the interwebs).
It appears the S550 is "rear surround" on or off. There is no option for a "single" rear speaker. I doubt the 500 has that option either. If you want, go download the manual.
Since Sony doesn't offer...
Well, after I read the manual for the S300(cause you owned it, assumed you were correct) and it has a 6channel, not 8channel. So you questions are moot.