If you're male, then it's co-ed (one side of the elevators is all male and the other side is all female)--there are no all male floors or houses at BU (or at least there weren't when I was a freshman). There is an all-female floor in Warren usually because of the overabundance of women who attend. http://www.bu.edu/housing/residences...alty/list.html
According to housing's website, Marshall 17 (which I'm guessing is B tower) is pre-med. Sometime people get placed in specialty housing for space reasons--I remember walking around the COM floor one year and seeing half of the residents listed as CAS '06 on their door. You may or may not be the only non-pre-med. There's no way to tell beforehand.
Each tower has 18 floors; however, they begin at the fifth floor (the first three floors are the parking garage and the fourth floor is where the escelators take you). I think the fifth floors of each tower are apartment-style rooms reserved for tower heads and faculty-in-residence.
"I am a political science major (who happens to love playing guitar) and I don't want to be surrounded by a bunch of study geeks who have no social lives."
No offense, but that's a crappy attitude. Don't start pre-judging people before you even get to college. There are plenty of us who have very heavy courseloads--whether pre-med, in SMG, in ENG, etc.--who manage to not be "study geeks with no social lives."