All of the current units in the mod are posted >
here< Necropolis. The turtle sub was dropped in favor of the Fulton Sub as the generic submarine. It happens that the British can also build a Hunley.
There are currently 512 units in this mod, and some custom units are still being made. Variety is not currently our problem.
Your definition of "the Steam punk era" could use some refining, though there is no authoritative definition one can point to. The Steam Engine wiki points out that there were steam machines that date back to the first century, and that the first piston steam engine dates back to 1690. There is also a category of Clockwork-punk that stretches the genre back to Leonardo Da Vinci and earlier.
On the other hand, there is also the whole "World of Tomorrow" genre with Googie architecture. This is often mixed with "Diesel-punk" which idealizes mid 20th century realism and Art-Deco.
For the purposes of this mod, we've tried to limit it to the Victorian and early Edwardian age (1837 - 1913 or so). Obviously, this is alternative history, so there are plenty of deliberate anachronisms (and some that aren't so deliberate -
c'est la vie). What's important here is that it's sort of a celebration of 19th (and very early 20th) century fiction. As I've said elsewhere, Moore's
League of Extraordinary Gentlemen was only the starting point. Your definition of steampunk extending into the 1960's may be based on Moore's
The Black Dossier that extends his story further into the 20th century, but that doesn't extend the whole steampunk genre that far out by any means.