If your home program/institution is some huge academic hospital, they may not care much if you apply to IM as a backup. We had people do that out of my class and our IM program didn't care that much...they had 60 IM interns and were used to occasionally gaining or added a resident in PGY2 or PGY 3 levels, and they had a lot of people coming in and out doing research years, etc. and had a big hospitalist service too to help cover the IM admissions. They also liked students from our school >> a lot of other med schools and so sometimes they would just take someone into IM even knowing it was the person's backup (for one thing, a lot of times the person would just end up staying for all 3 years of IM). But this was only for people who had done well in med school, well on the IM clerkship, etc. Most programs would care, though. I think it's a lot smarter to apply to separate hospital(s) for categorical IM and not tell them you are also applying derm or rads, etc.
Better yet, if you are a pretty competitive applicant, just apply to a lot of IM prelim years, to make sure you get one, and then if you don't get derm or rads, do a research year(s) after your intern year to try to make your application more competitive.