Caley - Welcome to the board!! As you can see, you ask 15 quilters what is the best combination and you get 15 different answers!!
Use what is readily available for you to start off. I agree with Rff1010 - give that thread a "yank" and if it breaks - toss it out.
15 years, when I began, I used Gutterman all purpose thread and a size 14 universal needle for everything. My preferences have evolved to Aurifil or Presencia thread and for needles I use only 12 (or sometimes 14) sharps. I prefer this combination as the thread is fine and the resulting seems presses nicely (imho). For almost everything I sew, I use a beige thread unless it is black or navy - then I dig out either Black of navy thread. Unlike garment sewing, I think that I can honestly say for quilting I mainly use a size 12 needle with an occasional switch to a 14.
As for what combination to use in sewing it all together, I longarm my quilts so I will let the other members who do free motion quilting on their domestic give you their recommendations. But if I were to stitch in the ditch, I would more than likely use the size 12 sharp again (size 14 if there was a lot of bulk in the seams) and choose a color thread that looked "pleasing" to the eye. It does not have to "match" - it could be a contrasting color. It really depends on what look you are going for and what floats your boat - lol - as you are finishing your piece. I have used Machine Embroidery thread, SoFine, King Tut, Guterman and Aurifil for top stitching or stitching in the ditch.
The final and my best recommendation is - Have Fun!!