Derm path is very competitive. It is available as a fellowship to both pathologist and dermatologists. You would have to start looking for a fellowship about 2 years prior to you finishing your path residency since most fellowships are not obtained through a match process many are filled very early. For example the cyto fellowship in my program is already filled for next year and has been for about 3 months or more. You would definately want to go to a residency program that has a good derm path program. Because your best bet for fellowship is always your own program or someone they know well. I am not sure how many spots are available. Check Freida maybe.
To do a derm fellowship you need only do the anatomic path part of path, but not a great idea if you don't get into a derm path fellowship. Best to do both AP and CP just in case.
Good luck