dakota - you could do something like that, each enemy group has an instrument attached to it. Quite easy to do in both FMOD and WWIse.
The kind of technical design you're doing, is actually an important part of game audio. We're always thinking of ways of making something stand out by thinking of different uses and ways of implementing audio. We are also trying to convey the mood and function of the story at that point in the game for the player. Sometimes when game play and innovation meet it really makes a difference. Sometimes however they can oppose one another. So prototyping is an important part of game audio programming / implementation to test if something will work before it's sent to engineering for resource allocation.
There are a lot of possibilities that could branch off from this, like music that automatically gains more instruments based on how tense a fight gets. Theoretically an entire song could be generated for different situations in a game. Have you ever heard of someone doing something like this?
Yes this is done quite a lot at the moment among other techniques and tricks. I've used intensity of a fight to drive music by adding more instruments or layers to make the music drive the game play with a more epic feel. I've also used techniques like music branching and real time mixing to create different moods out of the same music. A lot can be done, just think of the ways and reasons to do it.
It's cool when you start doing interesting things in the non-combat music. Here's an example of music that is generative to a certain extent, and with generative mixes to create a very tireless piece of music that can play for a very long time.