I think your solution will come by Old Glory.
They have a full range of mexicans so you may find lots of suitable figures:
pack PIM-029 mexican civilians
PIM-028 camp followers (the famous "soldaderas")
although the above are designed for the 1911 Pancho Villa invasion of US, I doubt that the fashion has changed too much, at least at the rural areas of mexico. Maybe for the capital you may also throw some civilians in urban clothing like those in pack BMM404 streets of Chicago by Blue Moon manufacturing.
Regarding policemen: I do not have any info for the subect, but I believe that the regular troops of pack PIM-16 will play their part. I suggest a light brown/khaki painting scheme for rural gendarmerie types. [If you go to use them you can add cavalrymen and MG with crews]
Old Glory website is here:
http://www.oldgloryminiatures.com/products.asp?cat=253&pg=2For urban police (policia urbana - if existed) use Anglian now Empress SCW Assault Guards (but I think those with shirts are more suitable)
You can find these figures from OldgloryUK, Miniaturicum from Germany and ofcourse direct from Empress miniatures.
Both Andy (OldgloryUK) and Michael (miniaturicum in Germany and fellow LAFer) are very friendly and can help you.
If not correct info, I think those minis have the Pulp feel.