You can use whatever letters you want, it's most important that they be the best they can be rather than from specific rotations or years. It definitely is a plus to have strong letters from attendings in fields you aren't going into. Also, letters from attendings who know you well are generally better because they don't just regurgitate your CV. PD's expect you to shine on the rotation for the field you are going into. They want to know that you have some exposure to anesthesia, so you really should get a letter from an anesthesiologist. Most programs want 3 letters, but ERAS lets you submit 4 to each program.
I used an anesthesia letter from an attending who knew me well, an anesthesia chairman's letter (not required but I used it because I worked with the chairman and he was well known in the field) one from peds (3rd year) and IM (3rd year). I got a lot of good feedback on interviews about my letters. A good letter from a surgeon is always well regarded.