I didn't even have a clinical letter. I suppose I could have gotten one but it wouldn't have been as strong as one from a pathologist, since i knew them better. I would say it is very important to have at least one letter from a pathologist, preferably one affiliated with a residency program. After that it's pretty much whoever can give you the best letter. A research advisor, if you are close to them, is generally a good bet too. A lot of people talk about 4 letters, I was always told three was plenty. Maybe that has changed. I think ERAS allows you up to four per program (not including deans letter) but that doesn't mean you have to have that many, particularly if they aren't adding anything.