How would a global superpower function in a world with thousands of ethnic groups with different levels and types of magic?

In this world, there are thousands of ethnic groups, each of whom are either a fantasy race, a nation of people with a magical bloodline, or a nation of people with a magical affinity. For example, there are thousands of breeds of werewolves, each with their own ethnicities. There are human ethnic groups with magics like super speed and telekinesis, and human ethnic groups with no magic at all. <br><br>However, in this world, global empires are also incredibly common place. In fact, the majority of the world is divided between about a dozen superpower nations (none of whom are nation states), all of whom have thousands of ethnic groups within their borders and use magic as their main military tactic. Obviously, a global empire with thousands of ethnic groups each with their own type of magic is going to have a hard time training their soldiers, as soldiers need to be trained in the magics they personally possess. How would such an empire function?