I use a 90/14 top stitch needle when I want sew use variegated thread for quilting. I agree that the 15-91 would be a good choice for FMQ. I have a 15-90 that is one of my favorites for FMQ. You don't want your tension on "0", you want the stitch length on "0". I had a 500a, never could get that machine to do FMQ it just said nope not doing this.
Sharon