No idea. As I said, the water I sourced them from went from frozen over to 80F+ over the course of the year. The tanks in which I kept them saw a smaller range of temps but still cooled down to max 60F in winter. I suspect they might have done okay longterm at any temperature within that range, but certainly don't know for sure. Expecting them to do well over the long haul at either extreme of their "survivable" range simply doesn't seem reasonable to me. They might very well exhibit the same sort of gradual weakening seen in things like Gymnogeophagus balzani, which apparently fall victim to various diseases if not given a cool resting period each year.
Again, I personally wouldn't mix fish in a tank where at least some of them were being forced to survive at the limit (either high limit or low) of their required temperature range. It's like keeping Discus and Goldfish together. They may not die immediately, or even soon...but they sure don't belong together. This just seems like low-level abuse to me; to use one of MFK's favourite silly descriptors: they won't be "happy".