I do wish there was a way to make different kinds of units count toward the archmage cap... Even if you can pull it off, it still doesn't seem fair to me. Maybe instead of having a unit cap, there could be some sort of penalty for overloading on Arcane units. For example, say an average civilization had an cap of 12. Amurites have an cap of 16, and Khadi have a cap of 18. Certain buildings or wonders can be built that raise this cap as well, alloing more units. Each powerful unit has an arcane cost. Powerful units like Archmages, Liches and High Priests have a cost of 4. Hero units like Henna or Govannon cost 6. Weaker units that are still powerful spellcasters, like Vampire Lords, cost 2.
How this would work is all these units that now cost magic points will no longer have a unit limit, I.E., a normal civilization could have either 3 Archmages or 3 Liches or 2 Archmages and 1 Lich, etc., without going over the cap. It is possible to go over the cap, however doing so carries a penalty.
Going 1-3 points over the limit causes slight magical turbulence within your empire, slowing down the speed at which spellcasters autogain EXP(About 70% of normal).
Going 4-7 points over the limit causes more turbulence, slowing it down even further(around 30%) and causes various things to happen such as random(negative) promotions being given to your units, plots losing food or hammers, unhappiness or unhealth in your cities, etc.
Going 8 or more points over the limit causes extreme turbulence, bringing EXP autogain to a halt, causing random building destruction and population loss in your cities, savage elementals spawning randomly in your borders, units randomly losing health or being killed, mana nodes being destroyed and mana changing it's type, etc.
Using this system, an average civilization could create 3 of these powerful units to start. Various buildings and national wonders can boost the cost cap by of 8, allowing another 2 units for a total of 5 for every civilization. One or two World Wonders can boost this an additional 6 points, allowing a total of 6 units(or 7 units going only slightly over the cap, with the only negative effect being reduced EXP gain). Amurites and Khadi would be able to build 1 more unit then this, for a total of 7(or 8 with only the EXP penalty)