Because you have said that the fabric is 'slippery shimmery' you need to be sure to use 5/8 inch seams. Because you want to save the faux cuffs you need to work close to them. Cut them off about an inch from the cuff. Cut off the excess length and re-attach the cuffs. You can indeed use a French seam to enclose the raw edges. The finished seam will be just a little bit above the cuff seam but will be unabtrusive because it will be parallel to the cuff. There is a tutorial on Craftsy what shows this method for shortening jeans that have traditional finishes at the hem line. They do not use the French seam because the fabric is different. Enjoy your new 'jammies.