article usage

Hi,

I have a question about the article usage.

When it comes to uncountable nouns such as meat, cheese, milk, fruit, cake, coffee etc., I have found out that they can be countable if they indicate specific items.

Here's an example.

I love chocolate cake. (compared with bread) --> Cake is treated as a singular noun because here, cake is a general term.

I need a cake please. (in a bakery) --> Here, a cake means one of the cakes displayed in the bakery where I am visiting. Here, the cake means the whole cake, not a piece/part of the whole cake.

I need a piece of cake. (at a coffee shop) --> Here, a piece of cake means a part of the whole cake. Usually, if we say, "I'd like some cakes", it means I would like some pieces of cake, not the whole ones. (In fact, my grammar book called, grammar in use explains this. It says, when people say "some cakes", it usually means "some pieces of cake", not the whole ones.

If this rule is correct, can I apply this rule to other uncountable nouns listed above?

Here's other examples.

I love fruit as a dessert. (compared with sweets). I sell fruit in the market. (compared with vegetables)
I am now eating some fruits. (some apples and bananas)

I like to drink coffee in the morning. (compared with tea)
May I have one coffee please? I need one Americano. (at Starbucks)

I love to eat cheese when drinking wine.
I came here to buy some cheese. --> hmm... in this case, even though I indicate some specific kinds of cheese or one kind of cheese, I still say cheese, right? I shouldn't say a cheese, or a few cheeses, right?

I love fish. (compared with meat)
Look at those fishes in the fish bowl! (there are several gold fishes.)

I look forward to hearing your answers.

Best,

Kay