Harlequins will be fine with a DG no reputation of fin nippers. The main fish with nipping reputations are serpae tetras, tiger barbs and blue tetras, but these are manageable in larger than normal schools so what I mean by that is where as most people would say minimum of 6 neon tetras you might consider a minimum of 12 serpae tetras this is because in the bigger schools the nippier fish will be so wrapped up in the hierachy of their big school they forget about the other fish, Im not 100% on this but I think that the nippier fish come from larger schools in the wild so they have to be a bit more voracious to stay near the top of the school if you see what I mean that way they are more likley to get to feed and breed. Where as the more placid tetras might be in the same numbers in the wild but rather than grouping as one large school they might spread out as 4 or 5 smaller schools if you see what I mean? Its all about survival of the fittest.. But yeah thats why I imagine some species are nippier than others. And thats what we have to replicate in the aquarium.
Wills