I would use the best letters you have and that best describe your skills/potential as a physician to be. It matters less if they're for advanced programs or if specifically tailored to prelims, since prelims know that you are only planning to be there for a year and that internal medicine is not your final choice of specialty.
That said, if you have an amazing internal medicine letter, do use it for a medicine prelim, even if it says "x will be a great radiologist down the line." I wouldn't bug the writer to change it, honestly. It coming from a med attending will carry a bit more weight. But I think a chairman's letter from a person who doesn't know you is less helpful than a very strong letter from a radiologist/anesthesiologist/pediatrician/etc.
Bottom line, don't sweat it. I did use a separate PS for my prelims, which I think helped at some places. Finally, apply to (and rank) more prelims than you think you might need - not too uncommon to have to scramble for these in light of a "happy" advanced spot match.