Minefields by their very nature are booby traps. Once you know its there you can avoid them or sweep them. In the 4th Imperium, the Scout Service would take a dim view of mining a planet, unless they did it themselves, say to protect a low-tech population from contamination, and would probably sweep it themselves. In the New Era if near Coalition space, the RCES would sweep the field, a good way to show the locals that the "Star Vikings" are not so bad after all.
First, sweeping the field may be a problem, low tech worlds couldn't do it, a TL-8 world could, but it wouldn't be easy, at Tl-9 advanced thrusters start appearing that would allow people better-easier access to space. Sending in a ship is self-defeating unless it has enough armor to defeat the mines laser bolts. A second way would be to shoot down the mine using a air to air like missile reconfigured for space to space use. A third way is to send someone in a EV suit to deactivate the mines. They would need space based personal transporter (rocket stake, jet pack) and a small craft to get them near the mine.
To avoid having your mine spotted easily it should have stealth or EM-Masking, a close in defence system, maybe a motion detector rigged to a laser rifle to keep anti-mine personal away.
You will also need both large and small mines. FF+S allows for interpolation and here I would use it! Nuclear Pumped X-Ray warheads range from 10 to 500 kilotons, but ordinary!!! warheads go as small as 0.1kt. These cheap mines would be sprinkled amongst the ship killers with orders to take out small craft, while the larger ship killers were ordered not to detonate against anything smaller than 100 D Tons.
How long do you plan to keep your mine field active? If you are targeting a single ship or say vampire fleet. You can skip this part. If you plan to keep the mine field up as a deterrent or road block you should leave replacement mines in the mine field. These mines would not be active, and would stay dormant until they picked up a mine detonating then after a short period, say two hours they would activate. The delay would let the ship that triggered the original mine to clear the field. This is not mercy just practical, you don't want ta 100 Ton scout triggering the whole field simply because the scout is to stupid to not blow up.
You might have to think about intelligent control for your mine field as well. Big ships are few and far between in the New Era but they do exist. A 500kt mine might not be able to penetrate the thick hide of a large ship so would be a useless to shoot at it unless you went for sensor/ antenna kills, but shooting at its support ships could make a difference. If you can't sink the aircraft carrier sink all its escorts and the carrier captain is going to feel very naked without them! Here a intelligent mine, maybe with a full sized ship's computer instead of warhead would work. A maned ground base, or defence platform (again there's one of these in BARD) could work and the defence platform could be of use in protecting the mine field. Also I agree with Tim nuclear power plants in my universe give off heat so I would also go with solar power and batteries. In case this isn't feasible there is a rule in Brilliant Lances that allows a ship to reduce its signature by one level if it reduces its power plant output to 1% so I see no reason that mines couldn't be designed to do this.
You might also think about placing your mines in the "junk belt" the area surrounding a planet that holds all the dead and living satellites and the other space junk that accumulates from a near-space faring society.
This is a good subject, its given me two ideas for adventures. First a Lancer crew is hired by the Coalition to lay a booby-trap mine field at a gas giant known to be a vampire refueling point in an effort to whittle down a vampire fleet. Second the Lancers are hired to remove a minefield that the Solee have set up to force a planet to join them.
Let me know what you think.
Roger