SST is a right side pipe. the fatty goes down and to the left(when sitting on the blaster) then winds around back to the right side. the SST just goes down and back up the right side. the length/point of connection is different for the sst and fatty i belive.
so, my rm125 has an fmf gold series fatty on top of a powercore, and it sounds awesome! you can intermix most pipes, unless the mid is a different size for some reason, which is usually isnt.