I have used 12So's, 12C1's, 12S3's, 3118-12AR's (12C1 clone) and 12S4's in Federal Papers. New Federal Papers come with 12S3's in them. My understanding is that ballistically, these wads are very similar. Depending on powder volume, you need to select the wad that gives the best crimp.
I don’t think that 20/21 grains of Green Dot will get you to 1250fps. You may need more Green Dot. When I need to go to 1250 fps using 1 1/8 oz. of shot with a Federal hull (games load), I prefer 23 grains of Unique.