Should I include derivations elaborated in other's paper in my paper's appendix?
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.