The Jem'Hadar are adults in about 3 days- to increase their numbers, the Borg need to be able to find people and assimilate them- and hope that works.
The Jem'Hadar were not to be poisoned- the White was. Mess the White's chemistry (ie randomly changing the chemical compound)- and then they don't have the necessary isogenic enzyme they need to survive- they go into withdrawl and die. Its not the same as giving someone cyanide. It would even take time to work- so you could send those soldiers against your enemies, and in their rage from withdrawl they'd be quite nasty..
The Founders can't have a "cell structure" per se to assimilate, or else how could they turn into smoke/fog and fire? They can- so if being assimilated, they could do that- or turn into a rock... etc.
All they would need to ram a Borg cube is one ship- on auto pilot, or with one pilot... and pack the cargo hold full of some sort of explosive... BOOM! No more Cube.
And as mentioned, Jem'Hadar shroud, and are good with bladed weapons, and are far more agile than the Borg.
Not that I'd mind of some Vorta were taken out... terrible, nasty things... but they really should kill themselves first- I suppose most of them know too much.