Ferrules used should match the tube, and the material of the nut and body should be at least as strong as the tube or stronger so that they do not deform during the swaging process. Compression fittings rely on a deformation of both the tube and ferrules, which is material dependant. If you want a joint as strong as the tube itself, where the tube wall fails before the tube slips out of the ferrules, you need the ferrule and tube materials to more or less match in strength- they don't need to be identical, but if the ferrules are greatly softer or harder/stronger than the tube, you get a different kind of joint and the failure mode may be different. So that implies brass ferrules are the right material to use with copper tubing. If galvanic corrosion is not an issue, you can use a stainless fitting body and nut with copper tubing, as long as the ferrules are brass.
If you are sealing 100 psig at room temperature, anything will probably work- including plastic ferrules, or metal ferrules on plastic tubing. So if your pressure and temperature are low, you can just jam a copper tube into a stainless compression fitting and swage it. It will work- but who knows what pressure that joint will fail at, or how it will fail.
For best results mixing of materials is discouraged.
The tolerances published for tubing partially control the tolerancing of tube fittings. Besides OD tolerances, there are tolerances in tube wall thickness, ovality and hardness. Use metal tubing materials that are softer than the fitting material. If you have different suppliers furnishing materials and fitting with different tolerances, the joints may not be perfect.
"The most important consideration in the selection of suitable tubing for any application is the compatibility of the tubing material with the media to be contained. Table 1 lists common materials and their associated general application. Table 1 also lists the maximum and minimum operating temperature for the various tubing materials.
In addition, Parker instrumentation fittings should be used only with stainless steel tubing, aluminum fittings with aluminum tubing, etc. The practice of mixing materials is strongly discouraged. The only exception is brass fittings with copper tubing.
Dissimilar materials in contact may be susceptible to galvanic corrosion. Further, different materials have different levels of hardness, and can adversely affect the fittings ability to seal on the tubing."
