The word "police" is rather special: It has no singular noun form. Something like that police over there is securing the scene would be incorrect. One would always construct sentences in the plural form like so:
The police are out in force today.
Anything done by the police will reflect on them.
Other words that take no singular form would include pants, trousers, scissors, and clothes.
Confusion arises because "police" is also used as an adjective. Consider these sentences:
A police department is housed in that building.
The police chief was highly visible at the town meeting.
In these two sentences, we are not speaking of "a police". You could easily remove the word from both sentences and they would make sense semantically and grammatically. Instead, the word describes the department or chief. It gives us context.
"Police" also has a verb form. You may encounter it like this:
The Boy Scout troop must police the area before they leave to remove any trash.
The verb means "to investigate, to search, to clean up". This certainly does fit in with a subset of the duties of a police department.
There is no rule determining whether or not a noun's plural form is the same as its singular form. As the comments point out, many of these words are words for animals. Another trend I have noticed is that loanwords from Japanese often don't change in the plural (two kanji, three Pokemon, ten anime), though sometimes they do (six ninjas, eighty futons).
Best Answer
I would say that the general guideline is, if you are mentioning two items that normally combine to form a new single item, then using "is" is preferred; otherwise, "are" is correct.
So when you say
Then I assume you are talking about bread that has butter on it. (For example, "Bread and butter is essential when eating a big bowl of spicy soup.")
If you say
Then i assume you are talking about bread by itself, plus butter by itself. (For example, "Bread and butter are essential for any kitchen.")
For your other examples, Word may say "is" is okay, but I disagree with it. ("Needle and thread" is a bit of a borderline case, since they do go together, but I would say that they don't really combine to form a new single item.) So sentences 3, 5, and 7 are wrong.
Since using "and" normally means that you are referring to multiple objects, if you're not certain about some particular pair, you're generally best off using "are".