X can be written as KS, GZ etc. but Z is still just Z and S is still just S. You cannot replace Z and S with any other letter but X can be replaced. And if it is possible replaced then you can write it as KS / GZ and X can be used for something else (softenning or enything else)
C can be written as TS. So by your logic, C is free to be used as a modifier and it is legitimate to propose CC, SC, ZC.