IMO/E, I wouldnt have a problem putting a Niger with a Volitan.
I had kept my 15" Volitan with a Blue Throat, Durgeon, Clown and one other...cant remember...and never had a problem. I never worried about the Durgeon or the Blue Throat, but I always kept a very watchful eye on the Clown.
Species that I would NOT keep with Lions would be a Humu and the other usual suspects (Queens, Undulates, Blue Lines...).