I currently have a Family Plan w/ 3 DroidX devices. Could I turn on the National Plus Mexico for the week we are down there and then go back to my original plan when I get back?
I want to take at least 1 phone with me and have data/voice capability.
Yes, you can change your plan for one week, just remember that making a calling plan change outside of the bill cycle start date will cause prorated charges and minutes/txt allowances. See more about it here. Of course, I cannot see what plan you have or would do using during that one week, so I can't quote prorated details. It's easiest to change your plan by signing in to My Verizon and go to "Change Plan".
Hope that helps! Have a great trip!