This is an interesting question, particularly because of this dichotomy:

> - This is a type of apple. (not *apples*)
> - These are two types of apples. (not *apple*)

I think that the construction of the form "two types of apple" sounds more than stilted; it just plain sounds awkward, and I would be surprised that it sounds familiar and normal to anyone (at least speaking for US English).

The idea that a plural form would be used for a class is actually not strange at all in English.  To express the fact that I like things belonging to the "apple" class, I would say:

> - I like apples.

I would not be able to use the singular to express this:

> - *I like apple.
> - *I like an apple.
> - *I like the apple.

Saying "I like apples" doesn't even imply that I am talking about multiple apples; one could say this, for example:

> - I like apples, although I've only ever had one in my life.

So, saying "I like all sorts of apples" seems to jibe perfectly with the rest of English grammar.

This means that the strange case is actually this one:

> - This is a type of apple. (not *apples*)

Saying "this is a type of apples" is definitely not natural or familiar.  It seems that, in phrases like "type(s) of X" ("kind(s) of X", etc.), there is generally number concord between the *type*-word and the class itself.  Why that is, I don't know.