I've heard of people getting away with shooting a string for years, but I like to change mine when it starts showing any significant wear. A little fuzz doesn't hurt, but a lot of fuzz or a broken strand, it gets replaced. I usually change mine at least once a year anyway, but it's a convenient for me.
Serving size will depend on the number of strands in the string and the type nock you use. 12 strands of Dynaflight '97 will be fine on a 55@29 (you could use less), serving size depends on the nock. If it bothers your fingers, you might consider a tab or glove with cordovan leather.
For 57#, you could go with 10 strands (maybe a little less, depending on your preferances) of Dynaflight '97 or 10-12 strands of 8125. A few more strands wouldn't hurt, if you prefer a larger diameter string--just cost you a tad of performance.
62XS is a braided serving material offered by BCY. I like it because it's tough, doesn't slip on the string easily (like Halo can), and I get a good release with it.
Chad