I believe that the "real" Nova-X isn't real at all, but an accident by the 3D Design team for "In The Beginning". So the story goes, during CGI for an engagement early in the Earth-Minbari War, a group of EA ships are seen getting trashed by a group of Minbari ships, and while they are intended to be Novas, if you look sharp you can clearly see the rotating sections of Omegas. The fan-decided reason for this, is that these were a small number of prototypes for rotational technology, grafted into the hull of Nova class ships, with an extension insert of course. It was decided that these were Nova-X, though I also have seen references to similar warships under Project Orion (also semi-fanon) which immediately preceded the Omega class.
It should also be noted, that the Shadow Nova (Nova-X) was a part of Den5's "In The Beginning" campaign.