Hello,
There is no any problems.
I suppose you need 2 groups.
1) Free group - dollars set as default currency, you can delete in and leave price filed also empty.
The same as on my image and then choose How approve user.
In this way you will get free group.
2) For paid group you can use MXN, we did not test it, that's why it is not mentioned in our list. You need create test account on PayPal and make payment. All must works fine.
Thanks,
Taras