Since the quilt is already quilted, I would have probably done needle turn for the applique. However since you've already ironed it on with fusible, you could just stitch about 1/4" inside the elephants and their ears to pin it down. The edges will fray a little, but I think that will look cute. Or you could possibly use a zigzag. Like you, I'm afraid that a satin stitch or other decorative stitch would not feed evenly through the machine at this point. Normally you would do the applique before quilting, but good for you that you got such a cute quilt made in time for the birth.