I guess to me what's conflicting here is what you just said; gamebalance vs. history. Like you said, nobody really wants to make the portugal AI WORSE off with the Boers (though I think they're by far the coolest addition option.
); whereas the spanish AI comparitively could really use it. But I'm not convinced that Mexico (New Spain) is really the best historical option here. You see what I mean? It's that key question again, history vs. game.