I have had an Ileostomy for 35 plus years, and I had a Gastrologist for anything that was/has to do with G.I./internal part of the body, symptoms, vitamin, loss of weight etc.
For thing like rashes or stoma help most places have a wound care person of some sort.
I have found that theses two specialties will work together or even refer you to another specialty if needed.
Research for best Gastrologist for type of issue is you may have, like Crohn's, Ulcerative Colitis, Cancer Etc.
Good luck and take care
jp