Nothing is absolute. You just do your best, make decisions and press on.
Acutally, in the past I started a thread entitled "How to get great LOR". Besides the flame war that entailed regarding LOR 'etiquette' (sp?), one thing I generally recommend is asking for letters from virtually every doc you have worked with, and I think the closer you ask to the time you rotated with them the better. You never know who is going to 'rock your world' with an awesome letter. Obviously titles/chair of departments etc carry more clout-use the best letters you get. I am sorry I don't know what "PI" is. If you meant a research faculty member with an MD/PhD or whatever, I think it depends more on what you are applying for. I would list my research and references separately unless you are applying for a research-heavy specialty or are headed to a research-career. If your application is considered incomplete in the heat of interview season, you are shooting yourself in the foot if you don't submit something otherwise they won't look at the app seriously.
One bit of advice I did recently hear recently was to set up a anesthesiology rotation, for example, at a very 'prestigious' location and try to get a letter from someone with stature, possibly even national recognition. Thereby increasing your odds everywhere including the same location. I never tried this because I didn't even hear about it until too late. Also, my rotations were relatively too late in the game to even consider trying to get a letter from someone like this, but it sounds like a good idea to me.
Good luck!
