A small fleet of borg is easily defeated by the dominion
Possible, but unlikely.
The Feds were essentially on the same technological level as the Dominion by mid to end of the War.
And the Feds had to send a fleet to destroy a single cube.
You are overestimating the Dominion and it's capabilities.
Borg are over-rated..if they have 'millions of ships' then why havent they taken over the galaxy yet? Because they are overated...DOMINION scientist are equal to federation and they would, as stated above, beat the Borg with some kind of virus..if a second rate engineer like Geordi and Wesley could take down the Borg, why not the Dominion?
They haven't taken over the galaxy because that way they would lose a valuable source of new technologies (meaning races that are sufficiently developed to resist them to a degree ... like the Federation).
Keep a sufficiently large pool of races in the galaxy for new and diverse technological discoveries ready for assimilation.
As for the virus in question ... it was never tested.
While it is possible the Dominion could definitely resort to extermination of the Borg with various viruses and numerous technologies like the Feds ... the Borg also have hundreds of safety measures to protect themselves from such scenarios ... which is why we never see them taken down.
Oh yes, they took a blow in 'Endgame', but while Voyager knocked out all of the Borg's TW hubs in the galaxy via chain reaction, problem is, the Borg can still travel through TW velocities independently.
They received a large blow ... but hardly one that would exterminate them.