I have been an alumni interviewer for several years.
Don’t worry about nail polish or heels. Just be yourself. If you typically wear heels, I would wear heels. Otherwise, don’t bother. The most important thing to do is to give a genuine representation of who you truly are.
To prepare for the interview, make sure you do some research about the school and ask some specific questions about life on campus and about the school. Talk about how you will contribute to the school and where you will fit in. Also, be sure to ask the interviewer questions about their experience and how/why they chose that school.