There is a very good possibility that subspace mines could be detected by the Borg easily.
Their sensors are much more advanced in comparison to other races.
Voyager was able to scan at least 2000 LY's ahead with it's Astrometric sensors when they encountered a region with no stars/planets that was about 2000 ly's long, whereas before the lab/sensors were invented, they had an ability to scan in a radius of 40 LY's (at least that's how far Janeway said they scanned for Borg activity when 7 picked up a homing beacon from the U.S.S. Raven).
It isn't just a matter of how much farther Borg sensors can scan ... it's also a matter that they have a way of detecting things regular SF sensors for example don't.
If the Dominion had issues conquering the AQ with a combined force of both the Breen and Cardassians (and the Dominion likely sent about a half of their fleet initially), I have issues with the assumption they'd put up a fight.
They'd essentially be in the same position as the Federation.
Send 1 cube ... sure, it will likely be destroyed.
Send 10 cubes ... well, it's game over.
Also, over the argument if the changelings could be assimilated ...
It's a questionable aspect, but there is a possibility that it could be done if modifications would be done to the nanoprobes that would acquire abilities to shapeshift along with the changelings into whichever state they morph into.
After all, the Borg can incorporate aspects of other biological species into the collective.
8472 were invulnerable due to their extremely powerful immune system ... but at the same time, they were from another dimension, and still susceptible to nanoprobes when reprogrammed to camouflage themselves as the 8472 cells (which is a temporary solution) so they have a shot at destroying the cells in question (actual assimilation would likely be possible with enough time to investigate the matter).
And changelings were still susceptable to biological viruses ... one of which nearly killed them entirely.