moritheil said:The default works the other way around. No abilities stack unless they are called out as stacking. DR and SR are redundant when they point out that they don't stack. This is a design principle.
I am not sure that is true. In fact the quote you put forward with stacking does not apply to Fast Healing since it is not a modifier to a check or roll. Since the rules quote you provided does not back up Fast Healing from not working concurrently perhaps you could provide one that does?
moritheil said:Also you may want to note how most templates that add fast healing are careful to say either that it improves fast healing by X, or (more commonly) something akin to "if the creature already has fast healing, use the higher value."
It looks like from the quote you provide here that when the Fast Healing of a specific source is meant to not be cumulative it calls it out specifically. Do you have something for the other side of the debate?