You can find a torque table here for various types of metal for small machine screws: http://www.federalscrewproducts.com/torque-chart.htm
I don't believe nylon has a torque value as it certainly strips easily. The torque numbers come from the manufacturers of screws and bolts, as they have ASTM standards for alloys and the materials that make up the screws.
Note that if one were to apply torque to a screw with a nut on the other end, then the torque applied to the nut also comes into play. That is, one has to sum the torque applied to the nut and the torque applied to the screw to get the correct total torque. If the nut doesn't rotate, the cartridge along with the headshell doesn't rotate, then the torque reading on the wrench (or torque screwdriver in this case) would be correct.
I must have a hundred+ small screws left over from cartridges. Some that came with the cartridge but never fit and I had to use screws from other brands. Some were too long, some too short, gosh I think I have every material of which small screws were ever made. In general, I prefer either aluminum, brass, or stainless screws. I have used nylon but only as a last choice. The screw material the cartridge comes with is my first clue on what to use.
I never torque cartridges, partly because I never owned a small & spiffy torque screw driver and partly because I am happy to just get the dang cartridge aligned without ruining it! I am sure I would destroy a cartridge sooner or later if I tried to torque them!