That, my friend is the strictest definition of "The Beauty Of Fan Fiction / SFC Modelling". I ~love~ this grey area. Ok, so..assuming you're right, the Terran Empire is effin' desperate for ships or altogether out of business in the Mirror Universe.
There ~has~ to be a Maquis / Resistance faction in there ~somewhere~. They don't sound to me like they're the kind of people who were born solely to roll over & play dead at the slightest sign of trouble. There had to be a "Worst Case Scenario" in the event the Empire's own ~internal~ security problems came back to bite them on the ass.
Here's the idea: if *you* were hurting for ships & personnel / tech, you wouldn't be splitting hairs over the *who*, *what*, *when* or *where* of the details, but happy as hell just to be able to get your hands on *anything* warp-capable at all. Armed or otherwise. You'd take what you could get & lay low with your supplies untill the enemy is arrogantly complacent & gets sloppy, thereby somewhat vulnerable.
You'd be asking for help wherever you could get it. The battlefields (in "our" universe) of the Borg Invasions & The Dominion War would be rife with raw materials. A virtual mall of potential Empire-worthy warships also on-loan to the Federation through the Mirror Kirk & Mirror Spock, to win the Dominion War on "our" side. The stuff I've read suggest they've perfected the transdimensional transporter, in the hopes of replacing key personnel authorized to divert Federation ships to the Mirror Universe as-is at this point.
There ~should~ be a major "arms race" at this point. Everything from Venture-Class Scouts to Sovereign-Class dreadnaughts due to the stuff I've read in William Shatner's novels (i.e.: "Spectre", "The Return", etc.) (Read about the specially-fit Defiant Class U.S.S. Monitor, aka: U.S.S. Enterprise-F) I'd suggest however that the Mirror Terran Empire Ships will bristle with more covert intel & more hostile weapons as they care little for civility, to them a veron-t disruptor is every bit as useful as a handheld Starfleet phaser. If they open fire, you're as good as already dead. The Empire's been backed into a corner. Not a smart thing to do to people with the mentality of a wounded badger at the best of times.
So...go crazy. You're in survival mode now. The Borg & Dominion of both Universes...the Klingons & Cardassians, all vying for a piece of the Alpha Quadrant. Design your ships anyway you want!
Talk to a user named "Thunderchild" at Treknology.Org's "Trek Creativity" Thread Forum, he designs awesome alternatives to Danube-Class runabouts (i.e.: The "Warwolf" Fighter) & other ships. You should find awesome inspirations there for CGI modelling Mirror Ships.