Should I take modern physics and classical mechanics in the same semester? Or is that a bad idea. And my concerns are mostly about needing to understand stuff from modern to understand classical. Thank you
They are quite independent - classical mechanics will teach you how to work with Lagrangian's and Hamiltonian's which are required for quantum mechanics; you will do a lot of partial derivatives, and hopefully learn about the calculus of variations.
Modern physics will cover a lot of - surprise! - modern physics! But it will emphasize the _physics_ and not the abstract calculations that are required for analytical mechanics or quantum mechanics. You should learn about quanta from the earliest physical experiments such as the photoelectric effect, the idea of photons, how a laser works, atomic spectra, electronic transitions, etc -- then some nuclear physics (E=mc^2 and more), and some relativity. It can be a great class.
I don't see a problem taking both as long as you are not otherwise overloaded.