In essence, mixing turtles from regions they would normally meet each other in should do no harm if proper quarantining and vetting is done. Making sure all your animals are wormed and parasite free is a good idea as well.
One thing to observe, though, is their reactions to each other. This goes for turtles of the same species as well. Make sure they are staying healthy and not abusing each other, and be prepared with every turtle you get to keep it separate from the others for it's whole life if that becomes necessary.