I'm not sure about the MS6 but the standard 4cyl will work, @blackerphotos swapped a magnaflow catback from a 4cyl to his v6 and i believe the only alteration that had to be made was the length of the pipe leading up to the cat
I put a Mazdaspeed exhaust on my 2006 6s (V6). It sounds great! It bolted right up to the back of the cat area and while I doubt it added any horsepower, it does sound nice, and the new exhaust tips look better too. Note that this is a Mazdaspeed exhaust designed for the regular Mazda 6 with the V6, not an exhaust designed for the Mazdaspeed 6 (with the turbo).