Re: So, you want a reason to not mix species?
I think that there is absolutely nothing wrong with telling a new hobbyist that they can't mix species. Would you tell a young child that smoking cigarettes/using the stove/using sharp knives/driving a car/drinking alcohol is ok? I mean of course it's ok for an adult, but the child doesn't understand why it's ok for adults and not for them. Which is why so many people use the "zero tolerance" parenting technique in order to avoid misunderstandings with their children. Once the children grow older they can begin to take on some of these activities, such as, using knives and cooking on the stove... New hobbyists (to any hobby) have to take this approach. (Yes I know there are people who jump into anything and everything head first and no harm comes of it, but those are "exceptions to the rule".) In the case of the dart frog hobby I just don't see the harm in drilling the "no mixed species" idea into their head so that one day when they have a room full of mysteriosus, pumilio, histrionicus, quinqs, etc, it can dawn on them that the only thing they haven't tried is a mixed species tank. My personal feeling, is that the patience and responsibility you have to exercise to arrive at that moment, are nearly as rewarding as finally creating a successful multi-species enclosure with animals whose husbandry and behavior you are intimately knowledgeable of.