in Motive there is the option to sort the cameras according to their serial number instead of the 'arbitrary' ordering.
Is something like this also possible via the Camera SDK? I have found the function
You are correct. The Camera ID of a camera is primarily a human readable number that can be displayed on the camera and/or in your software and UI. For USB cameras, their Camera ID is fixed based on it's physical connection. Now for Ethernet cameras, the Camera ID is 'arbitrarily' assigned as you suggest.
If you want to have your Ethernet cameras report Camera ID's in a certain order, create a list of camera serial numbers and pass it to SuggestCameraIDOrder. Following the call to SuggestCameraIDOrder, you should be able to call CameraID() on your cameras and those Camera IDs will reflect the order you suggested in your call to SuggestCameraIDOrder.