I can't agree more with the advice quantum and others have given.
For the sake of "authentication", I am a student at the Colburn conservatory and just finished up auditions for graduate school. Then again, I could be lying, so does it really matter what I say I am on these forums?
To think auditioning with familiar pieces doesn't matter is ludicrous. This is an audition. You have to find a way to separate yourself from the crowd. This isn't your debut performance at Carnegie Hall where repertoire is less important and most everyone will love you regardless. You are applying with A LOT of TALENTED applicants. I feel that it's wrong to think of how many applicants are applying with this academic year. Think of the professors you will be auditioning for, most of them have been teachers for many, many years. This only compounds the issue of submitting familiar works. Also keep in mind that you will have to submit a prescreen recording to these conservatories. It really doesn't help to play the typical works. Quite honestly, I wouldn't audition with Beethoven at all, instead I prefer Schubert. And while that is said, Chopin is one of my favorite composers, but I will always shy away from him when performing. All of his work have been performed quite frequently, it's honestly quite boring. Maybe a couple of Mazurkas as an encore (if I'm lucky enough!). Anyway...

Ultimately, you're going to do what you want and what your teacher suggests. And, ultimately, you should submit and audition with the pieces you play the best; however, if you audition with a lesser known piece and play it well, it really shows a lot about you. Most importantly, it shows that you take the time to research and study composers that are out of the common gaze.
It doesn't matter how many voices your Bach fugue has. The E minor fugue from book 1 is exceptionally difficult and it is only 2 voices. The inventions can be tricky, and those are only two voices as well.
Most schools will count Rachmaninoff as a 19th century composer although most of his compositional output was in the 20th century. But please please please, check with the schools directly (be sure to talk to either the director of keyboard studies or a professor of piano).