We know there are about 9-10 different alien types in the game, that each alien has their own discrete strengths and weaknesses (the Sebillian hardiness, the Andron ability to break through walls) and we know that each alien type has (depending on alien) 1-5 classes. This makes for variety within each alien type. However, will we see alien types mixing, perhaps in the mid-to-late game? I mean, this already happens in the terror mission with the terror drone, but will we see aliens mixing later on in the game?