The stuff in the Earth/Minbari War book was only really designed for a campaign set in that time and the scenarios in the book. It's entirely up to the players involved as to whether you allow them to be used outside of that period (some of the in-service dates stretch beyond the war). Official tournaments would normally only allow Armageddon lists, but (at least personally) I don't see any reason why you couldn't use them as long as your group had no objections. After all, the Armageddon ships are generally better than the Minbari War ships. Just look at the Hyperion Pulse Cruiser stats.