The first suggestion wouldn't be too hard to code, it would just tax the servers to do a checksum on the unit and setup a global module to check when a wizard hits a certain level. Best times for that would be to have it check after a battle exits and when a player logs in or else it would eat up latency a lot.
Another one that someone mentioned would be to ladder it based on tiers, but that really would be a pain to code. If the above is too much, hard cap them at 20 population cap, so to run 2 wizards they could only have 20 cap worth of troops and that would also mean issues with looting.
A 4th suggestion, which would also help reduce the power of both wizards and dragons, would be add a building to each race that allows you to research elemental resistances for units.