ARE there any 12 fighter dropships? The "Carrier" Carries 10 Clan fighters, but the Okinawa, Titan, all carry 18, while the Clan Titan carries 30. There have been references to a Union CV, but nobody's got published stats for it (that one would carry 12), so I'm kind of wondering if you're low-balling "Mid-Size" below the most commonly available carriers in the game-universe (okay, the Leopard CV's THE most common carrier, but if that's the issue, just limit everyone to Leopard variants in the 6,7,8 columns across the board.)
The curve for Inner Sphere design appears to be 6 (the Leopard CV, old-model and updated Overlord, etc.) followed directly to 18 for the Inner Sphere (Titan, Okinawa, etc.) with the "Heavy" class really belonging to the (Should be rarer than it is) Vengeance at 40.
While the curve for the Clans is 10, followed by 30. (Carrier, followed immediately by Miraborg, Clan Titan.) There aren't a lot of "Mid-size" range designs to look at/find.
What I'm getting at, is that if you're NOT a Clanner, you're going to be stuck with the Leopard CV in the 6,7,8 columns, or with Overlords if you don't want Leos.
I would submit that anything else that might exist, is probably very narrowly faction-specific, probably FWL....