Please explain what you mean by those terms, they don't mean anything to me (and I am British).
pluralsThe word plurals is the plural of plural. As in: I answered the question about plurals to the person who didn't know that the plural of plural is plurals.
Symposiums already is a (British??) plural of Symposium. The other accepted plural form is symposia.
The correct plural form of criterion is criteria.
Natural is an adjective so cannot have a plural form. It is from the noun nature, for which the plural is nature, that is, encompassing all of nature.