I searched YouTube to see what others have done for rear brakes and found a video showing a similar front swap (different color calipers) and the rear upgrade solution you suggest.
The rear upgrade uses your suggestion, the original S197 caliper and the larger 2013-14 GT500 rear brake disc. This puts 2013-14 GT500 discs all the way around the car.
Video Link -
One topic the video goes into at the beginning is brake balance. This is critical in keeping the car drivable and safe.
When you repurpose rotors front and rear from the same vehicle, it helps in maintaining brake balance if you can replicate the same brake piston area ratio, front to rear. This isn’t a sure fire method of getting the correct brake balance, but it will get you to a good starting point.
The 2013-14 GT500 calipers have 4 – 43.9mm pistons up front and a single 43mm piston in the rear. This makes the front piston area 4.69 square inches and the rear 2.25. Ratio – 2.04:1 front to rear. FYI 1.9-2.1:1 is the typical range for the vehicles I have checked over time.
The rear calipers on the S197 have a single 42.77mm piston with an area of 2.23 square inches. To maintain the original GT500 ratio of 2.04, the front brake piston area should be at 4.54 square inches.
I couldn’t find any data on the piston sizes for the 6 piston S550 calipers to verify the ratio. If anyone knows the piston diameters, please share. And if the data I am using is flawed, please correct me on that as well.