Now for the record I was more than aware that Southern Congressmen and Senators pretty much opposed giving equal rights to non-white Mexicans and hence they were opposed to All-of-Mexico if that is what it resulted in. On the other hand though there might have been more willing to to annex Mexico and Central America if it meant not giving equal rights and have those regions as slaveholding.
When you factor in that adding Canada would mean that they hold a disadvantage in this front, the Slaveholders would have been more desperate to gain more territory to compensate and thus they would be less picky in terms of annexation if it meant the balance of power can be maintained.
Yes, but if Mexico entered as an economic colony of the south, as you surmise, then the Mexicans would have been that much more resistant, especially when the southern imperialists began the necessary ethnic cleansing to get their colony the way they want it.