When someone sends me a session to mix, I generally assume they intended that everything is supposed to be included, but I'll tell you the secret to a great mix: it all comes down to good production. If they put pieces together that fit in the first place, the less you'll have to try and worry about what you are describing. That said, people go crazy worrying about this more than we ever did back in the day, and it's common sense. What supports the song? Is your kick or your bass the lowest instrument? Just go through and make decisions based on what is most important, and what needs the space. Balance first, EQ first to correct obvious problems, then see what you actually NEED to do.