To put a close on this, you CANNOT choose your AI opponent’s cars freely like you could in previous FORZA titles in the online lobby.
You only have options to set specific rules, such as drive train, year, etc. but you cannot click on the AI and choose car, this option has been taken away, which is a shame.
You can if you have the time to set up all the options to get the cars you want, but its just pot luck what the AI will pick and its extremely time consuming.