"Military" might imply a tool for political work, that is, centralized and controlled adjusting of national borders or the attitudes of neighboring nations. We don't really see anything political in the combat actions of the TNG Ferengi. They just steal stuff, and blow up other stuff that stands in the way of a good theft.
It seems unlikely that Ferengi combat vessels are in any way "military". They might sell their combat services to the highest bidder (say, the Grand Nagus) in a mercenary manner, but as you point out, the history from "The Magnificent Ferengi" suggests they don't.
A rather good defense against gunboat diplomacy is to hail the gunboat and tell the skipper that his payment for the engines is two days overdue... And the small print in the contract allows a better-paying customer (both the Klingons and the Breen are lining up) to come confiscate the engines unless suitable amends are made.