Any herp species can transer disease to any other herp species. All living terrestrial vertebrates, for that matter. We all have our own special brand of beneficial gut bacteria that helps us to digest our food, which has evolved over centuries to thrive in our unique biochemistry. Mixing species means they get a not-so-healthy hit of each other's particular strain of gut bacteria. At best, the foreign strain kills off some or all of the bacteria that herp needs to properly digest food and you've triggered some wicked indigestion. At worst, everybody dies.
Never mix species, particularly reptiles who are more prone to captivity stress than warm-blooded creatures! If you want a mixed species tank, get some fish.