I'm fairly new to the FMQ, so until I get better, I like thread that blends more with the fabric rather than lots of contrast. Variegated threads come in so many colors. When there are 2 choices similar, I generally opt for the lighter and have been happy with my selections. My favorite is Wonderfil Tutti 50 wt cotton. The only place I've found it is my LQS. It seems expensive at first, it's 1000 meters for about $10, but I've done a few quilts with the same cone and have 1/3 left. It goes a long way.
My 1/4" is much better on my new machine. It has a 1/4" setting and I can also adjust a little at a time. I had one of those 1/4" feet with the fabric guide on the side. My seams were usually off. I think the guide must have bent. If you aren't off too much, match the seams and pin and gently stretch while you so. I think it works best if the larger section is on the bottom.