Computer systems do not spontaneously break down. With the way DF is designed with hard cap on number of instances of certain dungeons, even if everyone was using only DF all that would mean is once your allotted number of Coil or whatever dungeons filled up, everyone else is stuck waiting in line. The system isn't going to explode from the fact that there are a lot of people in line waiting for Coil. So there's really no reason why adding Coil to DF would break anything. Now whether adding Coil to DF does any good? That I have no idea. It can easily be a ghost town like the normal mode Primals or Cape Westwind, or it might actually work. Like many suggested some kind of minimum item level requirement is most likely needed.
