The Mexican citizens that are Marines would be fluent in English and likely easily qualify on education. The delay is that they can't start much of the citizenship process until they are honorably discharged.
If they have met all the education requirements they ought to be sworn in as citizens the day they finish their Marine contract.
http://www.armybase.us/2010/01/foreigners-in-u-s-military-get-fast-track-to-citizenship/ claims some sort of fast track is already in place.
Would they not need a ‘green’ card to enlist in the first place? This would apply to every foreign national, not just those from Mexico. DREAM didn’t address this issue anyway AFAIK - it was strictly about those here illegally from any country.