Addressing Professors: Between Dr. and a hard place
Perhaps this specific point wasn't terribly clear in my answer to the question Theta30 linked, but to answer your specific questions:
Yes, you may address a professor of any rank (associate/adjunct/assistant) as Prof. X in most universities/depts in the US. In fact, despite not being strictly accurate, faculty who hold lower ranks (instructors, lecturers) are also referred to as Prof. X by students in some institutions.
I can't really give a straightforward answer to your second question as whether it would hurt someone's ego is mostly dependent on the person. I've known some professors who have a preferred title and take offense when anything else is used, and I've known some who are fine being called by their first name by complete strangers. In general if you want to be safe, you might try using the same title if one is applicable to both parties. E.g. if they both hold doctorates, something like "the research of Drs. Smith and Jones" or if they're both faculty members "the research of Profs. Smith and Jones".
The rule that I believe now prevails at my own institution (Binghamton University, one of the four doctoral campuses in the State University of New York) is that anyone with "professor" in their title (whatever qualifier--assistant, associate, emeritus, research, visiting, adjunct--may also appear) is accepted as "professor" (as a term of address and of reference) by faculty, staff, and students. First-naming of these individuals is permitted for faculty and staff only when speaking with others of their ilk in informal contexts. Similarly, associate and assistant deans can all be called "dean" (but this does not hold for the underlings of provosts or presidents), but will also be called "professor" or "doctor" as their credentials allow (ditto for others at higher administrative levels). Beyond that, "Mr" or "Ms" seems to be the norm for teaching assistants and professional staff in formal contexts. Students and non-professional staff, however, address each other almost exclusively in the first person, regardless of the level of formality, and are referred to either in the same way or by their full name depending on their (actual or presumed) level of familiarly among interlocutors.