While cross-mixing is a bit more complicated that what I want to tackle here, there is a big tip that will help you a lot. You are likely mixing against other instruments that can have their foundational frequencies (the frequencies that define the instrument) in one of the above bullet-pointed frequency ranges. You can either boost or cut one instrument’s range over the other’s range so the two instruments sound distinctly different instead of stepping on each other. It’s best to cut the key frequencies of one instrument instead of boosting the one you want to stand out. Less is more.