Mines don't have to be as powerful as they are in SE4, though - when was the Last time your ships were damaged but not destroyed by a minefield?
"A few cheap units", with the proper mine system (i.e. mines do less damage individually but you can have more in a sector, which incidentally can be modded in SE4
), would only serve to slow down a huge fleet, damaging a few ships here and there. (That's what we could really use with mines though - instead of all mines damaging one ship then the next, the damage should be allotted randomly so you end up with a bunch of moderately damaged ships most of the time!) It would only be large minefields that have the power to take out a large fleet.
And incidentally, you CAN mod mine-detecting sensors into SE4... just create a sensor with a detection level equal or greater to the mine's cloak level!