Markus, my strategy is (often) to expose to get roughly the same sky background values, as it's the background where the noise is most troublesome (least signal). When I'm feeling lazy, just the same exposure time
The response of the camera sensor to the different wavelengths of light isn't linear across the visual spectrum - have a look for the QE (quantum efficiency) plot for your sensor. You may be able to estimate what sort of exposure times to aim for from that.
The noise in each channel is independent of each other - but will be dependent on the QE and exposure time as above.
There will be a sweet spot, regardless of planetary/DSO...but the number of frames you stack for each will be vastly different (typically thousands for planetary!)