I bow to Sena's greater experience in regards to the pearl danios. That's the first time I've heard it, though.
As far as the tetras go, Von Rios are just like most tetras - stressed if not in a big enough group and some may be prone to nipping. My and my friends' experiences with Embers have been that they are totally chilled and peaceful, as well as slower swimmers (though they are fast when they need to be). I'd be worried about the fins on the Von Rio being a target for a betta and vice versa.
I didn't like embers much when I first saw them, but now I am addicted - they colour up really well when they get into a decent tank, especially with live plants, and just look beautiful. :)