I don't know, wasn't it like 60 years before the United States allowed Japan to build an offensive fighting force after WWII?
The Federation is not the United States, and the Cardassian Union is not Japan. While there may be some similarities to real world countries or organisations etc, does not mean they are based "identically".
One thing i'd say, is that there is ofcourse the possibility that in the 25th century, Cardassia may have applied for membership within the Federation. And thus, as Cardassians, they would likely want to continue to maintain a fleet of ships of their own. There is evidence of member worlds having their own ships or military services. Such as the Andorian High Guard, or the all-Vulcan crew of the T'Kumbra (note the lack of a USS in said name). The latter ship wasn't seen in canon either, only mentioned.
On topic: The ship is really shaping up quite nicely. It's like a futuristic, sleeker variant of the Galor class warships of the 24th Century. Will be interesting to see if you'll equip it with equally futuristic weaponry based on past Cardassian design.