If the government was truly destroyed to the point of being unable to rebuild itself until all the chaos and such wears down, I still wouldn't think gangs would go for surviving politicians, even if they weren't greatly protected.
Although at the same time, if you could classify what gangs already are doing as rebellion (breaking countless laws is pretty clear to be a refusal of obedience, I'd say), you could say their current rebellion will just carry on over.
Having moles on the inside to get information and do some dirty business, as well as controlling law enforcement via corruption has nothing to do with my point. :P As you've established (a few too many times >_>) gangs won't have any need for this in the event of a nuclear apocalypse... Unless they've already infected a government as badly as, indeed, Mexico and such.
In the event of nuclear apocalypse, governments controlled by criminals wouldn't be governments anymore. Any funds, supplies, etc. they possess they'd hog for themselves. Basically such governments would eliminate any person not bought out, merge with the gangs they're related to, and become a hub for said criminals to easily live while looting anything they come across, turning from a corrupt government into an organization of parasitical policy.
Or, if they have any true intellects among their control (if you can buy opinions, you can buy smarts), continue the farce that they're politicians and use the cover of the nigh-end of the world to snuff out anyone not on their side, and if a government ever becomes important in the social structure in the long future, they'l have complete control. An investment of sorts, really.