Rid-X is NOT necessary if you have a septic system. All it is is a form of yeast that naturally forms anyway. Experts will tell you that you really don't need it. What you want to avoid is bleach and other anti-bacterial products, which will kill the bacteria necessary for the breakdown of solids in the septic tank. Adding something like Rid-X after using bleach isn't going to help, because the bleach will kill the bacteria in Rid-X too.
Cotton is not going to hurt your septic, because it is biodegradable. It's going to rot away. The problem would be that until it does, if you do many rag quilts, you could at least temporarily clog your leach field and cause problems. I would think the problem would be if you're not using a 100% natural batting. Polyester fibers are not going to degrade, they WILL cause problems.
All in all, the laundromat is not a bad idea...