Minefields don't make much sense in GC. Remember, each hex is a parsec, that's 3.26 light years in length. Deploying mines over that large an area isn't feasible. The only way I can think of to make it work, would be to make a 'minefield deployment system' something you can build on a planet or a starbase, so that attacking fleets would have to deal with the mines.
I see no more problem with a minefield hex than I do with the various modules available for military starbases. It's no more ridiculous to have a minefield than it is a module which slows movement, or enhances missile/beam/gun effectiveness. In this case, we've already handwaved reality enough in other places to make that complaint about minefields irrelevant.
And it could simply be a module you add to a starship (both minelaying and sweeping).
Cheap fighters also are no substitute for minefields; fighters cost money to build and maintain, and are significantly less effective against capital ships in the GC universe.
Personally, I would urge people to think of these things less than the traditional stationary "mine", and more like an autonomous drone with passive sensors, waiting around to detect a nearby spaceship, then attacking it as a single-shot missile/gun/laser. That is, more like a torpedo or Anti-Radiation Missile (e.g. HARM) today than the mines of yesteryear.