I kind of understand what you mean: limit things so that you can focus on developing specific strategies to counter specific foes in specific situations.
However, mages, or units that use magic, take a long time to develop in skills etc.. I do not think it is possible to have a skirmish in which magic is used, since you never start with a mage. Perhaps starting with a mage could be made an available option?
As far as limiting units, however, that also limits strategy. I mean, I do not see much fun in cutting out the legs from under an opponent and then scream victory and start raving over my achievement after I kill him with arrows.
Point of that analogy: Each race uses its different units in different ways, limiting units really limits the capabilities of the AI. That is why there are different difficulty levels.
So, I think I can see where you are going with this, but I do not think it would be effective.