Isolation spacing in CPW-G
I'm not aware of an industry wide rule of thumb, but I typically think it depends upon the manufacturing technology and what sort of edge definition it is able to achieve.  With 50 ohm CPW over ground line, if the signal trace width is significantly smaller than the substrate thickness such that there is minimal coupling to the ground plane underneath (which would make it more like a microstrip line) the edge between the signal trace and upper-ground plane defines the characteristic impedance (Zo).   Perturbations in this edge will create changes in Zo, so I think it generally best to make the signal trace no smaller than 10x the edge definition capability.

For example, with 50 ohm CPW over ground on a substrate with Er=9.8 and thickness 20 mils, if the edge definition capability is ±0.5 mils and the signal trace is thus made to be 5 mils, the Zo variability will be between 48.8 and 51.2 ohms.