I think an interesting question, though, might be: what are the mirror Devidians like? Perhaps they're a force for good in the Mirror Universe! Angels rather than spectres... Or perhaps their abilities work in some unexpectedly different way.... Just tossing out some ideas!
I can see what you mean...that said, I don't know that I would literally alter their biology for another universe.
Oddly enough, as I see the MU, the Ferengi may be the force for good there. Their predatory instincts do not seem as strong as other species (my theory being that the Ferengi were not the apex predators on their world until very, very recently, evolutionarily speaking, and therefore behave more like small predators like a housecat or a ferret), and that may render them immune to whatever it is that has corrupted the MU to such a degree and driven the predatory instincts of so many species into a frenzy (one that at least as I see it, the Devidians would undoubtedly be caught up in--making them either the same as they are, or even worse). In the MU they seem to have become just bold enough to take a stand, but not SO much so as to become consumed by it.
At least, that's the way I see the MU.
Personally, I am a great believer in giving my characters a hard time. One of my literary role models is Alistair Maclean, who was very definitely of the same opinion. The protagonist of The Satan Bug, for instance, is gassed, half-blinded, and has his foot run over by a tank... and this is before the book even starts; when he reaches chapter one, he has real problems to contend with. Something to aim for, I think.
Hmm...I think that sounds a bit extreme. I definitely think characters should face challenges--serious ones. That said, I also like for there to be bright moments, and I am not comfortable going as far as, say, the new Battlestar Galactica. So, given that right now Alyosha's in sickbay after having had an encounter that for him is just as difficult as it would've been to make him face a Mirror counterpart, I don't really want to go there.