Essentially canon indicates there are two main strategic exchanges - the November 1997 massive strike against the US, and a weaker one in autumn 1998 but was primarily aimed at industrial centres in the UK and Italy.
We also know the very first tactical warhead exploded on the 9th of July 1997, but their use continued right up to and probably beyond November.
A few individual warheads here and there ouside those basic guidelines isn't just compatible with canon, but I'd go so far as to say encouraged by it.
A GM just needs to be careful any additional strikes don't unbalance the known situation in favour of one side or the other.
However, it's my personal view the US would avoid nuking Mexico and it's supply lines if they possibly could - there's enough fallout floating around already, and as Web indicated, the Soviets have itchy trigger fingers and a demonstrated willingness to use it.
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If it moves, shoot it, if not push it, if it still doesn't move, use explosives.
Nothing happens in isolation - it's called "the butterfly effect"
Mors ante pudorem
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