For civilians in a post nuke environment where all sorts of records have probably been lost making it potentially difficult to ascertain who's a legal and who's an illegal I'd say there is a strong case to be made for opening up citizenship to anyone resident in south texas regardless of post war status provided that they a) meet the criteria already established, i.e. be of good character, free of disease, etc, etc, b) agree to obey the laws of the land and c) pay their taxes. Break the law and you suffer the consequences (personally I think banishment / exile would be quite an effective deterrent in the post apoc world - play ball or you get kicked out of the safe area).
Groups of former military personnel who have turned marauder should be treated as marauders regardless of whether they are Soviet, Mexican, or American. But I think for organised units of the Meican Army who are still intact and functioning as a military unit give them an ultimatum - tell them they are not welcome in your territory and must return to Mexico or face the consequences - help them return if neccessary - perhaps they have no vehicles / mode of transport so truck them down to the border and tell them to start marching south. If they refuse your ultimatum then you could argue you have a moral justification for trying to drive them out by force.
And with that post, my working day is done...
cheers