exotic turtles

Question
my neighbor is a collector of rare turtles.he has a total of 50. all of them are in separate tanks. he has one big tank can they all be put in to one. if not can you please tell me which species must be isolated.
                          thank you for your time

Answer
I WOULD NOT combine 50 rare turtles under any circumstances. It is never a good idea to combine...
- different species
- animals from different habitats
- animals from different regions...

...and it sounds like he would be doing all three with at least some of the turtles. It is not a risk I would take with rare animals- I don't even recommend it for common species.

Now- which ones will 'play nicely together' within those rules? I'd have to know the species. Trying to make a list of all the turtles in the world and who gets along would take a really long time.