Personal thoughts on this....
(1) Dermatology residency programs *generally* only care about dermatology recs. Most require 3 recs and it's generally best to only provide the amount requested. I've heard of some programs that will throw out applications with > 3 recs because "the applicant wasn't following application instructions". It's a competitive field and every program is looking for ways to reduce the number of applications to thoroughly review.
(2) If you had an exceptional relationship with a non-dermatologist (e.g. longitudinal mentorship with non-dermatologist over years, who you've worked with clinically and on research) and think this person can write an "outstanding" recommendation for you, you could consider sending that in lieu of a (1/3) derm rec for your derm application. I would caution you should be careful with this though, as derms generally prefer recs from other derms rather than non-derm. If you go this route, I'd request your Dean review the recommendation letter before determining whether to include with your derm application.
(3) Prelim / transitional is more open in terms of letter writers. I'd suggest getting at least 1 letter writer in internal medicine for your prelim application though. Choose an attending who has worked closely with you in an inpatient setting (either core rotation or Sub-internship). IM docs tend to be pretty nice, supportive people and they will usually do this for you. I would *not* send this letter with your derm application.