I think that doing family medicine now would be fine. First, as an MD/PhD, the odds of you going into FM are very low. Second, FM tends to be more of a touchy-feely kind of rotation compared to others like surgery or even IM. Third, FM is so broad that it will give you a brief exposure to all the major areas of medicine.
Before you start any rotation, you should look into what the grading system is at your school. Some schools care a lot about shelf exams and weigh them heavily in your clerkship grade, while others are much more clinical evaluation-oriented. Ask one of the third years (MD or MD/PhD) who has already completed the FM rotation about what it takes to get an Honors in the rotation. Once you know what your school emphasizes, you should apportion your time and effort accordingly.