Don't overthink it too much, you'll drive yourself crazy. Think of all of the amazing, full mixes that were created with just guitars, drums, bass and vocals. Music isn't a game of Tetris, don't feel obligated to fill every frequency band. Note which frequencies speak most to what the track is and make sure those are rock solid, then just strike a balance. There's no blueprint to making a song sound right, but there are plenty of unconventional, counter intuitive mixes that sound great.
Thanks for explaining things this way.
This is the consideration, and ultimate goal that OP should get when studying.
There's a certain amount of overlap to instruments, in a way, that you cannot remove every little bit of what you don't want. It's some times best to leave a little bit of the undesirable parts there, because it still makes up the sound. It will balance itself out with other components that do take up that range.
One simple advice that i got from a noisia video is that "it's there, although it maybe almost non existent, the sound is still there."
Consider things as a blend of EQ and compression(individually, mixing groups, mixing song). Know how much range allowed to be, as it influences the overall picture of your mix.