I guess it's not possible because if you just skip the second slot and add the AI in the third, it'll act as the second one. (it doesn't matter if you jump to 8th spot, it'll be player 1 and player 2)
So, the only solution that I thought was... Make a mod with a race that has no monarch, like "Unknown" race. For example:
Player Aramon
AI Peter Unknown
AI Garrett Veruna
AI Gronk Unknown
AI Mark Zhon
The AIs with "Unknown" race will be invisible and do nothing. I think that's it, but then you'd need to mod your map or something to be playable multiplayer, and play with your mod singleplayer.
I'm not sure if those AIs will appear as "Defeated" but there must be a way to do it. Like, make an useless unit for the race, like a bird as monarch. lol