What kinds of corn can be nixtamalized?
Well, it looks like I found my answer:

Field corn is used for creating masa de maíz (see this link for my source). Field corn includes a few types of corn (dent, flour, flint, waxy). I'm not sure if all types of field corn are used for masa de maíz, however. So, if anyone has a more specific answer, feel free to give it.

I'm guessing really any kind of corn could be nixtamalized, however (but some kinds may be more ideal than others). Other kinds of grain besides corn can also be nixtamalized.