I can't agree more with what has been stated already.
Be very, very careful about asking for letters from anyone lower than a manager outside of the DO or MD letters.
Remember, most programs will allow you 2 letters so do not waste these letters. Please don't think that I am stating that Nurses or other health care professionals cannot write a good letter. What needs to be considered is how they can honestly evaluate you as a Physician, which for many programs will be only those in Managerial or Physician roles for non-academic letters. Hope this helps and good luck.