I just want to make a quick point here.
Although I agree that wild caught stock will be prone to disease and death at lower temperature.
You cannot compare this to tank bred individuals that have been bred to give several generations.
If they are bred at, for example, 27C for a few generations. Those that survive will be the fish that have slightly different anatomy which allows them to deal with the lower temperatures.
So over time they will 'as a species' become more tolerant to lower temperatures as by natural selection the fry that can't deal with it will die.
Making the alleles which code for certain digestive enzymes etc (that are sensitive the temps below 28C), more rare. And the ones that code for more favourable characteristics will 'over generations' become more common.
Having said the above, I would still go with Bolivian rams as they are my preffered Ram species. And can happily live in 24-26C no matter what source you get the from.