Should Shielding GND be isolated from circuit GND?
The shielding ground, by which i presume you mean the Earth and chassis as so forth, doesn't necessarily require a low impedance path to the circuit. You could short them together, but if you do make sure its done at a single point,  possibly close to the power supply. 

You should, however ensure that it's at a fixed voltage with respect to your system ground. A resistor to the system ground shod do the trick. It would hold the chassis at close to the system ground and would also provide a more direct path to earth from any noise or discharge the chassis may experience from the environment, than going through potentially sensitive circuitry.

I am assuming that your system ground is otherwise isolated from the Earth through a transformer.