I could imagine a lot of stress with different species being together. I'm sure there are exceptions, but I would go with a "don't do it" answer.
When I take Yuki out of her terrarium, which is next to Marimo's (They can't see each other while in them), he makes himself look larger, and shows a very aggressive attitude towards her. If I bring her nearer, he walks away, always looking back. She looks at him as if he would be a light snack after dinner...
This might be because she is far bigger than he and really COULD eat him if she wanted. And no, I don't put them near each other anymore. One time thing for a second already gave me my answer!
That being said, I will and wouldn't suggest ever mixing chameleons. I can't speak for pygmies or others who might have had success, but I wouldn't risk it. MAYBE if you have a HUGE free range...it COULD work.