Actually, if I remember my books correctly, there are entire cities of chaos, so presumably not everyone who serves Chaos does so in a combat role. There are slaves, concubines, and even chaos artisans.
As for where new CSMs come from, they just bash some normal guy over the head, stick him in some armor, and let the mutative powers of chaos do the rest.
Kind of like how any man can take a geneseed and have a small chance of surviving. Like, one in a billion. The training and selection process just make the odds much better