Nice fish but the ones pictured look like N.Marginatus. I would be concerned that your LFS is unable to do what is quite a simple ID...
My experience with Nannostomus is that the majority are pretty laid back (I've kept N.Espei, N.Marginatus, N.Trifasciatus, N.Eques, N.Unifasciatua, N.Mortenthaleri, N.Beckfordi, N.Rubrocaudatus) apart from the odd bit of sparing here and there. However I have always kept them in larger groups in tanks with plenty of floating plants and driftwood so there are lots of hiding places
I wouldn't recommend N.Mortenthaleri (coral reds) or N.Rubrocaudatus (purples) to most as they are the least laid back ones I have come across and the sparring is often quite intense
Good luck with the N.Marginatus, they are one of my favourites pencils