Hope this saves you many hours of reading and messing with txt editors to get the same results.
I would have rather been on the track than reading pdf's and many pages on the forums.

I did not change anything but the server name in the DS server.cfg.

Game Admin settings tips
First player to join the server is the game Admin and can set up the session as they wish.

To get the settings you want first you have to go in your game and set a server up as if you are going to host the game.

Click create online event
Set the setting for realism and rules and licence
in the car selection pick either Same class, Multi class or Any
now start the session once in the lobby click vote to start and as soon as it loads exit
the settings are saved and will copy over once joining the dedicated server but we will check agian.

Now search for the dedicated server (your DS name)
make sure you have your filter to show empty servers
may take a few refresh's to show up.

Connect to the server and while in the lobby next to Track selection click the arrows and edit to check the settings of the realism, rules, track and licence and change any if required as not all setting save.

Now do the same for Practice, Qualifying and Race session. (Time and Weather)
For free Practice just set the practice session to a few hours.

Click vote to start when you are ready.

If the cars are not right just disconnect and then reconnect again or change the track.

--Update for 1.3.0.0--

The server setting are saving now but there seems to be a problem with the cars saving.
From the main menu go into create online event and select a car from the class you are wanting to set up
Go into class selection and make sure it is set to sameclass or multiclass then select the classes and save it
You can check your other settings while you are here as we no longer need to start the game for the settings to save
Now you can connect to your DS and start the game and everything should be set as intended.

I picked LMP1, 2, 3 and 900 class and selected a LMP1 car
once connected to the DS when I sort the cars by class I can see the 4 classes and the WRX car that it always starts me with
I get the same results when I mix GT and LMP cars..

--Problems--
The only problems I have had is people not being able to connect once the server has been up a while this is easy fixed just stop the server and start it again and set up the next session via the DS console.