Hi Janice, if your quilt has a lot of fusible, you'll need a sturdy needle. A topstitch size 14 would be pretty strong and handle most threads.
If you use a 40 weight thread such as King Tut cotton your stitches will show up on the quilt well. A smaller thread like a 50 weight might not be heavy enough to get noticed. For ditch stitching to stabilize the quilt, if you need to do that, 50 weight is fine.
A word of advice here, based on my previous experience
. Test your threads on a sample first- when I used Rainbows I had to play with the tension quite a bit to get a good stitch. Make a quick sample with your fusible and batting that you are using and try it out.
I hope you post a photo of your quilt when you are finished. It sounds intriguing!
Kathy