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 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.
I think the rule (or one of the rules) you're looking for here is that some countable nouns in foreign languages are uncountable in English. This is apparent in nouns such as music. However, as far as I know, there is really no solid way to figure out which words these are — you just have to memorize them.
Additionally, another rule that pertains to switching between countable and uncountable nouns is one of plurality: In English, several words are uncountable when plural but countable when singular (e.g. hair).
Best Answer
How to join a countable noun with an uncountable one in one sentence, while expressing that one is "so many" while the other is "so much"?
The easiest solution would be to use the conjunction, and, to connect the two clauses together; however, the OP believes this would drastically change the sentiment of the original sentence. So I would like to suggest a few possible alternatives; my first is to use the adjective more, which can be used with both mass and count nouns.
My second suggestion is to use quantities of or amount of. Quantity can be used for both singular and plural nouns but preferably for inanimate ones. The standard rule says to reserve amount for nouns that can be measured in bulk and are uncountable. But I found instances where this "rule" was not always followed.