Although I've never tried it, I'm starting to suspect that chameleons have an aversion to eating frogs. There was another thread where someone (I think Sepioteuthis?) was showing photos of a tree frog living free-range with some adult Meller's with no problems. Any lizard of comparable size to the melleri surely would have become lunch.
As for mixing species, I've known of a good number of people with very large sized enclosures who've mixed individuals of various chameleon species together without problem. If the animals have enough space to have their own, perhaps slightly overlapping territories (with their own feeding, basking, etc sites), and don't identify the others as either competitors or potential mates they'll go about their lives just as they would in the wild; communicating without coming into physical contact with one another. Their sizes must be similar enough that none would ever consider feeding on the other. Now, I would not recommend just anyone do this. However, for the owner who has the time to monitor frequently, the experience (and intuition) to recognize when there's a problem, and the ability to change caging situations around, I think it provides a unique opportunity to witness more behaviors that singly-kept chameleons don't express very often, without undue harm to the participants. Again, not sayin' everyone should go out and do this. But, if you build a giant enclosure....