Hi, I'd like to know if there is any possibility to start a championship with multiclass grids. Seems this is not possible what is very unfortunate.
You can create one using the Custom Season Tool by selecting All Cars & Tracks option on the series (circled in blue in the image below). The Season Tool can be launched by right clicking Automobilista in the steam library and choosing the Season Tool option. In game select the matching All Cars & Tracks series on the main screen and enter championship mode where you should see you created custom season. After choosing your own vehicle you will have the opportunity to select the number of ai and the classes they come from. If you tick the set quantity button you can modify the number of ai for each series specifically. If not the game will allocate the ai accross all the series you selected.