Hello and welcome from Germany! To answer your questions: as my machine doesn't like all brands of thread, I go with the ones that run smoothly. My personal rule is to use both the same material for thread and fabric, so I use just 100% cotton thread.
To me, it is better, and I'm not talking about easier ;-)), to finish a project before starting another. While working on a quilt and esp. towards the end, EVERY OTHER, spankin brand new project attracts my attention, BUT I try to resist. Mostly with good results so far.
I look at it as a reward. For every finished one I may start a new one. Works for me, so this is just my 2Cents.
Most important is to enjoy the process, so happy quilting to you :).