Can you use a ScubaPro MK16 first stage and a S550 second stage for nitro?

You can use any 1st and 2nd stage combo with nitrox up to %40.

Additional comment

The contributor is correct that the Scuba industry as a whole considers no modification is needed to a regulator if the nitrox is not greater than 40%. There are some pros and cons to this. However, a titanium regulator is NOT considered nitrox compatible by many. So be careful with that.