Can DSP and GRBL be run on the same computer?

You have to create 2 device profiles
One for your Co2 and one for your Diode laser

Once you do this you can - with your device profile selected - navigate to Edit > devices and alter the scale if you wish (mm/sec and mm/min)

Regarding the layer settings, you cant isolate these.
If you used in your previous job a Co2 setting and create a new job - it will still have those settings.

Yes, you need to add your C02 as a new device. Then you choose which device you want to use. Each device has it’s own settings that activate when you switch to that device.