There is a subreddit dedicated to 18650s where people make power banks and the like, https://old.reddit.com/r/18650masterrace/ . Off the top of my head I remember reading about making sure you use the same set of cells together so you don't get unwanted current flow from highly-charged cell to low-charge cell, but I'm sure there's other details too
Absolutely, I’ve browsed that sub a bunch but frankly I trust the opinions here more. I’m using matched reclaimed cells that I’ve tested to make sure they’re still healthy.