programming an AI capable of doing the kind of evaluation necessary to allow it to pick appropriate scales, pretenders, and magic paths is basically impossible.
I think your only option is to start a game with all human players, and design each pretender yourself, then set all but the nation you wish to play to AI.
to my understanding, some ppl also use mods to give AI extra stuff in their capital, or their pretender extra stuff; anything to make up for the fact that the AI cannot actually evaluate and choose proper units.
The last point brings up an interesting option. Would it be possible to simply program the AI to recruit and compose only certain types of squads? That way its pretender could be designed, and then the AI programmed to only recruit units and compose squads that its design was made to use. Better than what it does now.
At any rate, your better off just forgetting about the AI, IMHO. Jump in an MP game. Although some ppl do find a lot of pleasure in playing SP 'challenges' and perhaps role-played SP; if maybe that's your thing. Still, this is by and far an MP game.
EDIT: if JK actually does make an AI that can do the evaluation necessary, then watch out,
Skynet will be coming online soon.