Can an open package of sausage lead to contamination of other foods?
While there is some question as to the wisdom of keeping or eating long kept fresh sausage (which has a shelf life of a couple of days in the refrigerator), it cannot magically reach out and contaminate other foods unless it is in contact with the other foods, or dripping onto them. 

This is one of the reasons food should be covered in the refrigerator--it minimizes the risk of getting dripped on (the main reasons are to prevent odor transfer and drying out).  

You should also ideally have raw meats stored at the bottom of the refrigerator where it is coldest and they would not be able to drip on any other foods even by accident.

Micro-fauna and pathogens are everywhere, in the air, on surfaces, and so forth.  The presence of he questionable sausage, even older, doesn't change that fact.

If your other food is properly stored (covered, refrigerated below 40 F / 4 C), its shelf life is not going to change because the sausage is there; it is the refrigeration that protects it, although it will go bad itself in the normal course of events.

Now, as to whether you and your roommate should have a discussion on what you feel is appropriate refrigerator etiquette and usage, that is another question that cannot be answered here.