First of all, I am not a vanilla bean expert, but here is a bit of what I know. Many love Mexican vanilla as it is a bit more robust and more pronounced in flavor. It does impart a different taste, so it is not just a matter of decreasing the amount in comparison to your normal vanilla.
One thing that is crucial though with Mexican vanilla is that there are different types. The higher end is safe and is what bakers love to use, but one needs to watch for the lesser quality version, which can contain a component that is actually harmful and banned by the FDA. I am very careful where I buy my Mexican vanilla. One indicator can be if you see vanilla beans in the ingredient list. However, that ingredient list has to be factual. I err on the side of caution, but that is me.
That aside, I love to test, so if you feel that you have a good quality Mexican vanilla, I would do a small batch of your favorite recipe and test it. I do not know of a ratio exchange for one vanilla over another, so testing would be my personal solution.
Hope that helps! Someone else is bound to chime in with more information to help.