Thank you all for the suggestions! I think I'll try finding a few neutral tones of Glide, since they appear to be larger spools, just to start. And it looks like there's a few Gutermann mixed color packs. I'll check my local shop when I visit this week
The cotton on cotton rule seems to follow the same idea as using cotton for warp threads when weaving. The cotton warps can hold a higher tension without stretching, so when the fabric relaxes it won't bunch or curl. Meanwhile the weft can really be anything, and I normally use wool or wool-nylon blends, which are pretty flexible. I'd guess you want to pick cotton if you're making something that's going to carry weight, like a bag or what-not. I'm super curious now!
Thank you SallyS for the book suggestion, I'll definitely look for that book (great title!). I'd love to take some classes, jeez... but at least there's YouTube.