Many foods are not good for dogs, We use a list that we have on what the proper people foods that are "safe" for pooches (I think we got it from the AKC web site). And many Mexican foods are off that list!
I can scan and copy it for you if you wish.
Melons, green beans, almost any kind of cheese except jalapeño are good for dogs. We often look at this list when we provide table scraps.
What kind of shells are those? We love mex food at our home