Yes, we can only setup AI opponents with tools that exist in rFactor2. Right now its either adding number of AI in menu before going on track or adding AI by "Add AI" button one-by-one once you are on track. Maybe some more useful option will be added in new UI that is being in development right now, we'll have to see.
Right now to ensure consistent AI opponents from race to race I use 2 methods:
1. If mod has enough unique driver-carskin pairs defined with its opponent filter you can set AI number in menu to number_of_mods_cars-1. For example, some F1 mod has all 22 cars from some season represented, you select one car and set AI number to 21, AI will be always the same. This works for Enduracers mod also, if you select any car then you can count in opponents filter how many cars this car category has, subtract 1 and set number of opponents to resulting number.
2. For mods like S397 GT3, S397 Endurance I set opponents number in menu to 0, write car numbers that I want to be AI somewhere on paper and then add them one-by-one when on track by "Add AI" button. Its not very easy, because if I make some mistake in race settings and go back to menu to correct it, then I have to add them on track again. But it is what it is...