Good question. Any cable will work on the FD7 but there is a reason why Fiio chose a pure silver cable to match the FD7. FD7 is a highly resolving well balanced pure beryllium dynamic earphone. Silver properties matches up perfectly with the FD7 as it brings out all the technical aspects of the FD7, stage imaging, instrument separation, timbre, tone and detail. Using copper is ok and it will really come down to preference but you end up giving up a slight bit of that transparency and technical ability of the FD7 for a slightly warmer smoother take on the FD7 sound. I can see how some folks will be ok with that trade off. If you're going to use copper. You can certainly use a nicer copper cable and the treble nozzle on the FD7 for a different take on the FD7 sound signature. So the idea there is the counter the slight smoothening of the treble region with copper cable using the treble nozzle which opens up the treble end a bit and in return you can get a more organic sounding FD7 and a fuller bass end than what it already is but then you loose that silver transparency for the FD7 in the process. Something like a hybrid cable with copper and silver mixed will work even better. Yes I did try the Penon mix on the FD7 but in the end it was the AG8 that won it out.
The AG8 is different than most silver cables on the market it is thick by design so there is no loss of note weight if anything it adds a bit yet has that highly resolving silver ability at the same time. It enhances stage better than most silver cables as well.
The FD7 deserves a much higher more resolving copper cable if you're going to use one like a higher end UPOCC copper or crystal copper cable will be better match up with the FD7. But again if you go back to the stock cable or something like the AG8 the stage, imaging and detail will change a bit. Since the FD7 has them tuning nozzles and it is an mmcx. There is some flexibility to really hone down the FD7 to a person's tonal and technical preference. A warmer take on the FD7 a more bassier version a more lusher version all that can be possible. But bringing out the best detailed FD7 with the best stage possible is what silver is all about.
Cables are a personal thing a bit of a way to lean a sound to your liking so it really will depend on some experimentation. Nothing wrong with experimenting with the FD7 to see how you like a cable match up. I tried a bunch of cables with the FD7 and I always came back to a silver variant. Again its stock cable is one of those rare instances where the manufacturer actually matched up the sound properties perfectly with the included cable. I bet it was a design choice. You don't design something like a pure beryllium flagship dynamic and have that not be a design choice. You don't think Fiio tried all their cables they produce on the FD7. I bet they did and many more to get the cable it came up with.