I would suggest sending: 1 science professor, 1 research, 1 dentist, and 1 community service. This make you look very well rounded as an applicant because it shows that you are capable academically (sci/research), you would be a decent clinician (in the opinion of your favorite dentist at least), and that you are compassionate (community service). I got the feeling in my interviews that the last was the most important, since almost every applicant invited for interviews has good science teachers and dentist letters.
Also, I think 7 letters is a bit much. I would stick to 3 or 4 really good letters. I would assume they look for quality over quantity. I would also research the schools you are interested in and tailor your letters towards what each school likes. Your application to private schools (especially if you apply to schools with religious affiliations) would really benefit from this.
Good luck!