This is a difficult war because the founders are Shapeshifters so i dont know if they can be assimilated, But the Jem'Hadar would have a problem because the are addicted to the enzyme Ketracel White.
I have to go with the Borg in a Borg v Dominion scenario. Though, if this is a Founders v Borg discussion and not a Borg v Dominion discussion, the Founders lose again. The Founders can't pose as Borg - the Borg don't care about looks. They'd stand out as much as anyone else.
__________________ In case we find ourselves starting to believe all the Anti-American sentiment and negativity, we should remember England 's Prime Minister Tony Blair's words during an interview. When asked by one of his Parliament members why he believes so much in America , he said:
"A simple way to take measure of a country is to look at how many want in... And how many want out."
You should read the Destiny books. The Borg are hardcore in that. They send something like a thousand cubes not to assimilate, but to wipe out the Federation.