Fun!

I had a hard with with "collections of nouns are called" (even though what I know what declensions are) because of a lack of context -- I answered "noun classes"

https://en.wikipedia.org/wiki/Noun_class

because that is another kind of collection of nouns that has something important in common grammatically. :-)

I might suggest "collections of nouns that use the same endings" or "collections of nouns that use the same inflections" or something, in order to be less ambiguous.

Tibi gratulor et filio tuo fortunam bonam exopto.