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.

Solution 1:

Your search for a rule is admirable, but alas! doomed to failure.

• The plural of fish is fish. Unless you're differentiating between species:

The smaller fishes are more affected by ocean warming than the larger.

Or if you are a mafioso, in which case you say

Vinnie sleeps with the fishes.

Of if you're a theologian discussing the miracle of

the loaves and fishes.

Different species form plurals in unpredictable ways. Both tuna and salmon are their own plurals. Species that end in -ing, like the ling form their plurals by adding a final s, except for grayling and herring. You just have to look it up here. However, if the fish name is the name of a special at your restaurant, you might hear a waitress call out

I need two salmons and three tunas!

She means two orders of the salmon dish and three of the tuna dish.

No matter how many you have in a bowl, you only have fish, never fishes.

• Cakes is the plural of cake. It never means pieces of cake:

  Some cakes have frosting; others have icing.

• The plural of fruit is fruits, but only when you're talking about different varieties:

  Some fruits -- bananas, apples, kiwis -- are good for you. The rest are not. You always eat some fruit.

• Drinks are the typical nonountable nouns, except when you're talking about varieties or individual servings:

Whiskeys are either blended or single-malt.
Give me two whiskeys, two scotches, two beers.

Waters has an additional plural as the naturally occurring water in a location, so during your vacation, you

take the waters at the spa at the hot springs

• Cheese follows the variety rule. If you have three cheeses on your cheese plate, then you have three different types of cheese, even when you have six pieces of cheese total. Same with milk:

I make three different nut milks in my blender -- walnut, hazelnut, and almond.

The plural of beef is beeves, but it's only used to describe individual animals, meat-on-the-hoof, so to speak.