Ruben,

I was thinking about how you can find your cousins in SFG Mexico. Either phone book or churches. For your ancestors, have you looked at the family history centers that the Mormons operate. I go there all the time. Let me send you their website.
www.familysearch.org
You can look up names and it will be linked to a microfilm . You can order the microfilm and go view it at the family history center.
Let me kow if this helps!
P.S. I was looking at some message boards and the name Arias shows up a lot coming from Spain.