Another issue with keeping different species together, even if they aren't toxic to each and come from the same area, is that you are putting them in a lot closer contact with each other this way, without them deciding it. So even if they aren't aggressive to each other out there, it could change once they have to share a small terrarium. I like the person who sits in the cubicle next to me, but I probably wouldn't like her as much if she had to share my cubicle all the time.