From memory only and this could be wrong, but I believe if you save up a favorite setup you can tell the ai in the player.json file to use it. I haven't done it in a while, so that could have changed.
Sorry, "Fixed AI Setups" is not the way to do it and there should be no need to edit the json file to assign setups to AI unless "Fixed AI Setups" was set to true. As I recall, highlight an AI in the STANDINGS window, switch to the GARAGE, select the setup you want the AI to use, and then click the AI SETUP button.
Emery, do you have to do that for every AI car that's listed in the standings? Or if you do one, it applies to all? Do you have to do it every race then, too? Or does it remember from session to session?
Yes, it remembers from session to session as it's just a file saved where setups are. You can copy the file created to other directories, but I don't remember what it takes to rename them or if they simply have subdirectories... having a look at the setups directory contents will probably make it clear to you after you assign a couple. Assigning slightly different setups is also a way to spread the AI out more than is possible with just the talent files.