I used 2 derm letters and a research letter. They don't care. PGY-1 programs will rarely even read your recs carefully. Certainly not prelim medicine programs -- transitional years might just because they can be more selective, but I didn't feel like I was at a disadvantage for not using medicine letters. Just remember that most PGY-1 programs will interview you soely on the basis of your numbers. If you're competitive in derm, you shouldn't have any problems, regardless of where your letters come from.
I used 1-2 medicine, 0-1 surgery (because I had already asked for it), and 1 dermatology...doesn't really matter unless they have specific requirements which you can check their websites for, I recall a program or two requiring a medicine chair letter, more than 1 medicine letter, or 4 total letters