How to assimilate the Dominion:
Convince a founder to shape-shift into a borg drone. Connect him to the hive mind. Use him to gain the relevant knowledge about the founders. Make him go to the great link and release fine-tunned nano-probes in it.
The assimilated founders: order all vorta/jem'hadar to report to to the nearest cube/assimilation chamber - and to eliminate any who refuses among them.