I had a group of 18 Hemigrammus bleheri (the commonly-seen species of "rummynose") and 7 Petitella georgiae (the so-called "false" rummynose) together in my 115g Amazon riverscape, along with some 50-60 Corydoras representing 12 different species, all wild caught, and the temperature was 24.5 C (76F). In the summer, during heat waves, the temperature did rise close to 80F (27C) during the days but went down a couple degrees overnight. This caused no discernible issues for more than a decade.
Long-term temperature and temporary temperatures are different things. The ranges usually given for a species on reliable sites intend mid-range for long-term care with the lower and upper limits being temporary limits with which the species should be able to manage.
Going forward, I would certainly invest in two heaters that will be set around 75-77F (24-25 C) year round. The summer increases should not be an issue, just ensure good surface disturbance. I do not recommend any attempts to "cool" the water, like bags of ice, frequent water changes, etc, as these will only result in far more drastic fluctuations within a short space of time and that can be detrimental to most fish.
Temperature is critical for fish because they are ectotherms, and depend upon the temperature of the water in which they live to maintain their life processes. Temperature drives the fish's metabolism, and extremes cause serious stress if not worse.
The diurnal variation in water temperature in the Amazon basin is not more than a couple of degrees, born out by numerous studies. And seasonal variation is almost non-existent; while the sudden downpour of rain will be a tad cooler, by the time the rain passes through the warm atmosphere it is not that much of a temperature drop. And then it hits the warm forest floor and is heated, and this is continual because the breakdown of organics occurring 24/7 creates heat. Something similar occurs in an aquarium; the water is drawn down into the substrate, heated by the biological action of countless bacteria breaking down the organics, and the warmed water rises back up into the tank to the surface (warm water like warm air rises, cool water/air lowers) and slowly cools to repeat the thermal cycle. This is sometimes hardly noticeable if you insert your hand into the water of the aquarium; it will always be cooler at the substrate than it is near the surface, and by a degree or two.
