You should be able to select the linear curve for all raw images, from any camera, if you are using C1 Pro. Not sure why it's not available for all cameras for your use, I tend to select it quite often so I know it's available.
As previously mentioned, if you want it to be the default, you just have to make your preferred curve under the preferences, however I believe as I use it the preference are per camera type, so you need to select it and save it for each type of raw file. If there is a way to make it the universal default, I am not aware of how, but it might be possible.
Paul C