grammar - When can uncountable nouns be countable?

I have a question about uncountable nouns that are used as countable in certain situations.

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

Here's an example.

1. 
   

   I love chocolate cake. → The term cake is treated as a singular noun because here, cake is a general term.

   
   


2. 
   

   I need a cake please. → Here, a cake means one of the cakes displayed in the bakery where I am visiting. Here, cake means the ‘whole’ cake.

   
   


3. 
   

   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, Grammar In Use by Raymond Murphy, 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 are some other examples:

1. 
   

   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)

   
   


2. 
   

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

   
   


3. 
   

   I like fish. (compared with meat)
   Look at those fishes in the fish bowl.
   There are several gold fishes!
   I need one salmon and two tunas.

   
   

However, some uncountable nouns are always uncountable as far as I can see.

For example,

4. 
   

   I love to eat cheese when drinking wine.
   I came here to buy some cheese. → 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. Am I correct?

   
   


5. 
   

   I love meat.
   May I have some beef and pork?
   Some pounds of beef, a pack of pork... → Here, I can't say “I need those beefs and porks”, can I?

   
   


6. 
   

   I usually put milk in my coffee.
   I need two cartons of milk. → I can't say “I need two milks” unlike “two coffees", can I?
   I need one vanilla-flavored milk and one strawberry milk.--> In this case, though, I kind of feel that I can count them because it states a more specific kind of milk.