Scorpion Part II
Admittedly after Voyager's second incursion into Fluidic Space:
Kes: I can hear them they want to talk through me. They say we've contaminated their realm.
Janeway: Tell them we had no choice. We were only trying to defend ourselves.
Kes: They say our galaxy is impure. It's proximity is a threat to their...genetic integrity.
Janeway: Tell them we have a weapon. A devastating weapon that can destroy them at the cellular level. If they don't stop their attacks on the Delta Quadrant, we'll be forced to use it.
Kes: They said, your galaxy will be purged.
The word "we've" would seem to be the key. Does "we've" mean the Borg, or does it mean Voyager and the Borg.
The next important part is that our "galaxy is a threat to their genetic integrity." That would strongly support the idea that all the life forms of the galaxy are a threat and referrs back to out galaxy needing to be purged.
So purging of the galaxy was their intent very early on because of threat to the genetic integrity and its proximity to theirs.
You cannot negotiate with anyone who believes that, until you prove you're powerful enough to defeat them or hold them at bay.
Voyager had the know how, the Borg had the capabilities.
Picard would not have liked it, maybe even consider it a last option. But allying with the Borg was the right thing. Unless, as was said, Species 8472 was completely rewritten.
If Species 8472 thought the galaxy was threat from the moment the Borg entered then Picard could not have negotiated a single thing with them.