Quick answer: It would be useful to you to differentiate between harmonic styles and histories, I'm somewhat surprised they did not emphasize such notions at Juilliard: Wagner is a romantic, (not post- as say Reger, Strauss, etc.); similarly, Mussorgsky is part of the Russian romantic nationalism, not neo-classicism (and these two are very different!). I don't mean all this as censure towards you, but you would not have these questions if you read a little less, and listened quite a bit more. If you did, you would be able to pick periods of music and harmonic systems easier I believe.
Anyway, Goldsmith just uses a lot of discords freely during the terror-scenes, but 95% of the music in the film is tonal if memory serves. I think you're confusing atonality with chromaticism, but the music of the film in question, if I remember correctly is still only slightly chromatic (compared to, say Stravinsky). A good book about the development of harmony in the last 100 years, is '20th Century Harmony' by Vincent Persichetti. It's not what you'd call current, but for you and the periods you mention up until the '60s I think would serve you well! Even more fundamentally, Piston's or Schoenberg's relevant books would help you a lot. Forget set-theories, integral serialism, and jazz from a theoretical point of view, until you know your Mussorgsky from your Shostakovich comfortably.
I hope this helps.