The current "100 mines per sector" limit seems rather arbitrary; here's an alternative way to curb uber-minefields, borrowed from Stars!
Mines have a decay rate! Each turn, some percentage, say 2% (of course moddable in the data files and modifiable by racial traits, mine hulls, mine components, etc.) of every minefield, spontaneously detonate / run out of supplies / whatever. (Perhaps if they spontaneously detonate this detonation could be detected by enemies, alerting them to the presence of a minefield?) Anyway, so suppose you manage to build a 1000-mine minefield. You don't have 1000 mines for long, though, because you lose 20 the next turn, and just in order to *maintain* the minefield, you have to launch 20 new mines!
There could always be some minimum value like 100 for which no decay occurs, so smaller minefields wouldn't need to be maintained and they would work like in SE4.
The same logic could be applied to satellites and any other kind of units that are deployed in space in large Groups for a long period of time... though it's a bit less evident why a satellite would spontaneously self-destruct...
(it might run out of supplies though)