The extra ship classes are from the
Neostandard list (link is to the Wiki page). It's not actually a mod, but just a reference list for shipset creators and modders to work from. If a particular shipset doesn't have a separate image for a Neostandard class, it will substitute a second choice from the stock images.
BTW, the most recent version of Space Empires IV Gold is v1.91. There's also a megapatch to convert "classic" or Gold to Strategy First's SEIV Deluxe; the highest version of Deluxe is v1.95.