When FMQ you're asking the machine to stitch thru a sandwich, whereas when regular stitching, just two pieces of cotton. Therefore, you may need to adjust the tension, among other things.
Needles types and sizes can vary ... depending on the fabrics (regular cotton or batik, heavy or light cottons, type of batting, # of seams you will be going over, and # of layers of fabric). Quilting, embroidery, and topstitch are some I have been using. And yet, I know some who manage to do an exceptional job, just with Universal needles.
You didn't mention what type of a machine you are using. Most FMQing the stitch length will not be set, as you have the feed dogs down and control the length with how you move the fabric. However, if you have a Bernina with a BSR, yes you will be setting the stitch length. Length is a personal preference ... I know some who stay as small as 2.0 and onwards and upwards from there. It depends what I am doing as to where I set it. I do a lot of practice for the project before I decide what I want.