I, too, probably wouldn't stitch a true flat felled seam on denim with the Skyline. A mock flat felled seam would be fine. On my 12000, I use wash away Wonder Tape to hold seams in place and stitch with the zipper E-D foot on the "single" even feed foot. It's the perfect width to ride right on top of the seam. I move the needle position to the left or right and align the edge of the foot with the seam. And if it's really denim, a jeans/denim needle works best to avoid skipped stitches or needle breakage.
Alina has a good explanation of using the Wonder Tape for this seam.
https://alinadesignco.com/2017/03/27/how-to-sew-flat-felled-seams/
Jan in MD