Anal Stenosis, I think, is commonly seen in newborns or infants.
This needs treatment by itself.
A adult person having "Anal Stenosis", if he/she is passing stools normally, then he can, as well go for Colonoscopy.
Colo-Rectal surgeons do surgeries, like complications Crohn's disease or Ulcerative Colitis or Megacolon, resection of Colon cancer etc.
Colonoscopy/Endoscopy is a different speciality where the physicians get training in looking, excising small polyps, taking biopsies etc. They are not trained in surgery. If the lesion is beyond the scope of Gastro-Enterologist, then it has to be dealt with by a surgeon. Colo-rectal surgeries, some of them can be done endoscopically, in an operation theatre, just in case, if anything goes wrong, they should be ready for Laparatomy (opening of the abdomen)