Minesweeper / Mine-layer
"Damn the torpedoes full speed ahead!" -Rear Admiral David Farragut US Navy 1864. Mines are one of the simplest and most effective defenses yet devised by science. They are cheap, easily stored, and frighteningly effective.
In Noble Armada it is inconceivable that the fleets of the noble houses would not deploy mines as a cheap fixed defense during wartime. Mine would be incredibly effective as a gate defense. Gate are huge constructions, but compared to a solar system extremely tiny. This means any Noble house knows exactly from where an enemy will emerge into his territory, the very fact of which would make Gate assaults one of the most violent battles a naval officer is likely to see. Mine also could be placed in gaps around asteroid belts as well as barriers around forts to prevent boarding actions.
To deliver these small but deadly cargo's and to counteract them, Houses need Mine-layers/sweepers. These most likely would be converted Galliots who have exchanged there troop compartments for mine storage racks and deployment equipment. Such ships could lay several patterns of mines and have the equipment to remove them from space, making them both a defensive and offensive vessel.
A few ideas for mines.
I see three general types of mines. Magnetic Iron mines, Cheaply made fission mines designed to detonate upon detecting the steel hull of a starship, such a mine would explode when any vessel even a friendly ones who ventured too close.
Second, IFF Mines, these more complex mines, possess electronic Friend or Foe recognition in the form of a radio code, which when received would allow said ship to pass through them unharmed, but would still remain effective against any enemy ship without such a code.
Third, and most expensive of all, Advanced Seeker mines, these would have advanced detection sensors able to properly identify an enemy vessel and actually move "abet very slowly" the entire patten of mines towards the enemy.
Rule wise, I would recommend a "pattern" of mines to be a template placed on the board which was between 3-5 inches in diameter. Any vessel or enemy vessel passing through it would be attacked. It would be assumed each patter represents dozens of mines so the pattern would not be removed after the first ship passed through it. A minelayer most likely would be able to lay down 4-6 patterns before exhausting its onboard supply. The removal of mines, could be accomplished by use of a special action, failure would mean the ship would receive an attack.