Is "négo" an accepted abbreviation for "négociation" in Canada (Quebec).
Négo is indeed a commonly used term to shorten "négociation". It is not an abbreviation in itself; it is a short form.
It is also commonly used in the plural form, "négos".
In the case of the expression/slogan you are referring to, I suggest that you keep in mind that a slogan is used for its "punch" power and the fact that it could be used to be repeated in the streets for a protest. Linguistically, it's not an excellent form, IMHO, but it conveys the message very quickly.