Ferrules, Fittings and Tubing - Material Selection
The short answer is like material goes with like material. Match stainless steel ferrules and fittings with stainless steel tubing, brass fittings and ferrules with copper tubing, and plastic fittings and ferrules with plastic tubing. You will head off the problem of dissimilar materials fighting each other, and you will be assured that your ferrules are going to adequately swage your tubing.