Should I include derivations elaborated in other's paper in my paper's appendix? I used results from derivations of a theoretical physics paper in my paper. Is is appropriate for me to include the derivations with a few adaption specific to my work with citations like: 'most of the derivations can be found in paper XX' or should I just cite the final result of their derivations, without writing an appendix section elaborating the derivations? 

a little more to ask : is it considered plagiarism if I write down all the derivations with most of them from their paper and a few adaptions even with proper citations?
It's standard to write "The derivation can be found in X and is included in the Appendix for the sake of completeness." It may happen that the reviewers suggest to remove the Appendix but in this way the presence of the derivation should not influence the overall suggestion of the manuscript. 

You may well give other reasons than "sake of completeness" (e. g.  self-containedness, slightly different derivation...) but some reason and proper reference should be given.