Assemblage can always fit in mixed media... Yeah it really can fit in mixed media but what doesn't

You can always say that assemblage is the same as collage. Well, they are similar but not the same.

Collage are several clips pasted together and assemblage, is the three-dimensional or sculptural equivalent of Collage. You can say it's like a sub category of collage.

Here are some great assemblage artists [link] and two great collage artists [link] [link]
Maybe after viewing their work you can have a better idea of what I'm talking about

"Collage & assemblage requires an uncommon degree of inventiveness, discrimination and even sophistication. When junk is to remain not merely a rearrangement of junk, but transformed into art and, possibly, a thing of beauty"

I really think that the collage category should have a assemblage subcategory.