The Stirling was not designed to carry torpedoes. That requirement was in specification P.13/36 which produced the Manchester and Halifax. The cells in the Stirling were designed for 2,000 AP bombs.I have no evidence other than if a 1930s plane can carry an 18" torpedo internally it generally isnt an accident, US, German and Italian designs had to carry torpedoes externally. The Hampden could carry bombs or torpedoes and so could be considered for both. I think it was the size of a torpedo that governed the "cells" in a Stirling bomb bay, that screwed it up completely for carrying bombs like a "cookie".
The Hampden was not designed to carry a torpedo and was modified to do so. A recall a post on this website showing the modifications.