One of my letters was from an IM dept chair and I seem to remember it being mentioned once or twice on my interviews. The rest were EM. If you think it would help your app, it's never a bad idea. I found the best way to approach it is to just be candid with the letter writers. I think I even said to the IM guy something along the lines of "Do you feel comfortable writing me a strong letter of rec to an EM application? I value your opinion and think a letter from you would carry a lot of weight, etc.. ". Most of my letter writers were candid back and you can easily tell that way if they are going to write good letters or not, for the most part. Virtually all these guys will write letters and are used to people asking them. You need to have a close enough relationship to feel comfortable being candid because hey... this is your application and you don't want them putting something in that that's going to screw your chances. Otherwise, I see no problem with an IM letter but obviously you need EM letters to comprise most of them. My letters were a point of conversation in at least half of my interviews, so good letters def carry some weight. I had one that basically said "If you guys don't want him, we'll take him." that I knew nothing about until interviewing with my 1st choice program who happened to mention it and made me very glad that I had meticulously approached each letter writer and tried to optimize my ability to get a strong letter. Good luck.
