There's a lot of variation from school to school. Most require at least 2 letters from science faculty (some like LSU-New Orleans go as far as to require 3 from professors teaching science lecture courses). Some require a peer letter, some require or recommend letters from non-science professors. Look up what each school you plan on applying to wants and plan accordingly (or plan your applications based on what you can get).
You definately want at least one non-professor who is an MD, esp if you shadowed a physician. That way you at least 'know what the field is about', which is what they want. After that, a couple from professors that can speak of your academic potential.
to be safe, line up a couple of science profs, a non-science prof, a supervisor for an EC you did, and a physician you shadowed/worked under. also research adviser(s).