I've no idea on ACO range quals (but IIRC, weren't CWO's only permitted to run short ranges?) but from a regular\reserve standpoint iaw: PAM21, all RCO's had to hold the minimum rank of substantive Cpl.
Of course, in that world, the range quals available to your bog standard infanteer, were SA(B)90 and SA(A)90. For the latter, one had to be a minimum of substantive Sgt. SA(E)90 (which I also held) was for acting as a Safety Supervisor on Field Firing ranges.