Its easier than you think.
Create a System for this county. Everything can go in that one system, P25 and all others.
Then create groups for each of the entities (County, Cities, etc.)
Then enter the channels as you would for any other type of system for the groups.
On that one P25 channel, for the SO, you would choose Digital for the Audio Type, setting the mode as Auto.
For all other frequencies, the audio type should be Analog and the mode should be Auto.
Auto is kind of a catch-all. After you get it up and running okay, you can tinker with setting the mode to NFM and see if you see any improvement. You may or may not.
If you have programming software, such as FreeScan or ARC-XT, with you being a premium subscriber, you can down the most of these things and fine tune the settings as described above.