For tens of thousands of years reptiles survived in their isolated geographical regions. They developed in their genes specific antibodies for resistance to bacteria, viruses and environmental differences. Their bodies adapted to conditions that were specific to their isolated area. With changing land forms, elevations, temperature, humidity, and food sources available; they further became even more specialized. Some were kept in their specific regions by mountains ranges, vast rivers, while others were separated by the vegetation that were their homes, protective cover, and food.
All the changes were slowly occurring and were a benefit to their survival. Some of them overlapped and lived nearby in a different micro habitat, or niche.
These changes affected resistance, and susceptibility.
Chelonia differ for example in their susceptibility to bacteria shell infections - due to B.chitinivora. For instance under certain circumstances, the Eastern Painted Turtle, Chrysemys picta, rapidly develops shell lesions while other species (Red Eared Slider, Pseudemys scripta) may carry shell damaging bacteria into a group housed situation without themselves having visible shell lesions.
Compared to terrestrial tortoises, aquatic chelonians such as pond turtles, sliders, wood turtles and box turtles appear to be more susceptible to mixed bacterial and bacterial/ fungal infection of the shell. These slow to Heal / and Diagnose infections can be avoided with proper quarantine techniques and avoiding the mixing of turtle species. The potential for parasites and viral diseases that once were only indicative of one species are currently being studied as epizootic diseases throughout research centers. ( The Florida Boxie has been moved to areas that they never inhabited. Now exposed to a fatal virus that lives passively in the Gopher Tortoise; thousands have perished.)
Note that while mixing different species may not affect each and every individual; consider also that these illnesses may not immediately present itself. These agents attack when an animal is stressed.....immunity is down. Zoos for example had the worst BoxTurtle outbreaks when mixing species.
For so many more than the few reasons stated above; I would advise against mixing species.
Respectfully,
Ric