Interesting topic that would benefit from some real science. It seems everything we have is based on anecdote and Disease issues mostly applied from animals totally unrelated to chelonians. I do mix aquatic species, but not from different continents or from differing major climate differences and very careful about introducing a new and especially wild caught animal without thorough quarantine and checks. I do not mix tortoise species at all, however, I am not sure if I have good reason with tortoises raised in the same captive environments and similar habitat / diet needs.What is the word on mixing turtle species? I've always mixed tropical aquarium fish from different parts of the world, and that has generally not created any problems for me. I've always known mixing tortoise species was a disaster in the making. So where do aquatic turtles fall in this spectrum? I know many people have large ponds with several turtle species. Are things just different because its an aquatic environment?
Short answer - I see no issue in mixing species with aquatic if you meet each one's needs in the enclosure you have created.
LONG ANSWER-
I see four major issues in this regard - Habitat. Diet. Parasites. Disease.
Habitat - this would be the ecological niche the animal will best thrive in. Temperature, humidity, photo periods, and physical structure of the environment. = what we create with our enclosure. Most tortoises from similar climate zones do well in very similar setups with a few exceptions like the pancake tortoise. However, I feel aquatics are more differing in this regard. Deep vs shallow water. Great swimmers vs bottom walkers. Fully aquatic to mostly terrestrial. Rocky bottoms to sandy bottoms. Open water vs cryptic in plant cover. Open baskers vs cryptic or floating baskers. Etc, etc. My best tortoise enclosures look almost identical for sulcata, or leopard, or star, or radiated, or hinge-back. However, a cooter or a spotted turtle, or musk turtle or a box turtle from the same river system needs very different setups. So I feel it is important that the enclosure in which we put these animals accomplishes what that animal needs.
Diet - Most chelonians are opportunistic feeders. They will eat whatever they can find. Tortoises have evolved a much more similar dietary need relying heavily on plant material so lower protein intake and gut flora to digest/handle high fiber. Aquatics can be more specialized. However, almost all will thrive on a well balanced higher-protein pellet diet. I don't understand why so many folks seem to automatically disdain commercial pellets. A ton of research and science goes into mixing the required nutrients, vitamins and minerals into a readily eaten and liked pellet. The variety of foods that would have to be given to ensure the same availability of nutritional value would be overwhelming. Gut flora is a big part of diet. So lots of talk is about the gut flora a particular species has and therefore the diet that is best. However, I hear little talk of how the gut flora of any animal changes dramatically in total profile to the environment and diet subjected to. The gut flora adjusts to what is available. That is a big part of why I believe captive raised individuals, started on the right diets, do so well compared to wild caught or even poorly started individuals. With aquatics, I have not had one species that will not thrive on a good pellet diet. They get other things for fun and treats. Probably more fun for me to see them go after something that really gets their attention. But I have turtles that I am on at least the 8th generation of offspring in my pond. At least 98% of their diet is pellets. The same with my koi. An animal that can and has lived to be well over 200 years old thrives on commercial pellets alone.
Parasites - Most are benign and indeed a part of the gut flora. Some are problematic in captive environments with a direct life-cycle where re-exposure and re-infection continues to build if animal density and cleanliness is not watched. The few of great concern are going to be introduced from outside or wild caught sources and of especial concern in mixing species in that regard. Most parasites are quite species specific. OR - more correctly certainly Class and Order specific. The ones that can effect our chelonians by introduction by and intermediate host are more limited by diet to the chance ingestion of snails / worms/ or a dead frog for example. But these are normally controlled by good hygiene and reduced chances of reinfection as the intermediate hosts required are not always contaminated nor that available on a regular bases with our chelonians. Especially if we are feeding a pellet based diet. So again, with captive raised individuals from a good keeper, parasites are a rare problem. I have never had a parasite problem I have had to treat with any aquatic turtle. Mixing species is not the issue. Where the individual come from is the issue in my opinion.
Disease - How much of this is overblown hype due to mammal experience is a good question. Most all chelonians seem extremely resistant if not immune to most all diseases we normally fear. For example in his book The Crying Tortoise, Devaux talks about his finding that Sulcatas have no natural pathogens whatsoever. They don't naturally get sick! The diseases we do see in chelonians are normally the result of husbandry issues, not infection from other sources. RI, impaction, gastro enteritis, shell rot, vitimain/ mineral deficiency. Not because a sulcata gave it to a radiated. Or a cooter gave it to a map turtle. Even the dreaded salmonella that so much ado was about and still is - chelonians don't "get" salmonella! They never are affected by the disease. They don't carry it because they are infected. Salmonella is a ubiquitous bacteria. Most anywhere bacteria is allowed to grow due to poor hygiene - you will soon find salmonella. Put a turtle in 1" of water, add food and poop and let sit for a week without cleaning. That's what you saw with the turtles sold with the little plastic bowl and palm tree. That is a perfect bacteria culture mediaum. Do the same with some chicken you bought at the store, or an egg. All will give you a salmonella culture. The way a good keeper keep their chelonians today, there is more risk of salmonella from your supermarket than from your chelonians.
sorry for getting carried away - but this touched a topic of great interest to me!!