I have damsels, a clown goby, and a scooter blennie and my damsels totally ignore them, even when the goby and blennie are swimming around a lot. The damsels never even bothered them the first day I added them. But I might have just gotten lucky, I don't know.
Purple Nudibranches are difficult to keep because they eat one specific type of algae and once that is depleated they will starve. I read somewhere that the blue velvet is even harder to care for, had something to do with the head shield but I can't remember what it was or where I read it.
I have had good luck with Berghia Verrucicornis (Aiptasia Eating Nudibranch). He was hiding in my live rock when I bought it 2 months ago and I just found him about a week ago.
So obviouly he didn't need any special care.
And they eat Aiptasia that's a plus. And Aiptaisa is something I can go buy at the pet store if I am running low (which I just had to do). So you wouldn't have to worry about starvation.