Yes, I've heard of it. You have the definition correct. You will hear in the advertising industry because it sounds like a combination of advertising and mixture. If you do a Google search for "admix advertising," you'll see this. You will also see some entries for food processing where ingredients are mixed.
Yes I've read/heard it, though it's not a terribly common word. I seem to think I've heard it in contexts like this: In recent years there has been a significant admixture* of Eastern Europeans to British society.