I would agree that any of the fish from Uruguay would work as far as temp goes.
Buenos Aires tetras would be a better fit than Lemons, because lemons are northern Brazilian warmer water species.
I kept Australoheros, and Gymnogeophagus in my none heated tanks. Or if you are not trying to be geographically correct, high altitude Mexican live bearers, or species from China would work such as Rosy barbs.
Australoheros"red Ceibal"
Gymnogeophagus quilero
Gymnogeophagus sp Paso Pache
Xenatoca eiseni, a cool water Mexican live bearer
and Gymnogeophagus balzani, with a cool water cory from Uruguay (C erhardti)