grammaticality - Should I follow English conventions, or write what sounds better?

How a sentence sounds when read aloud or in your head can often "sound" different for each individual doing so; however, I was reading details regarding the usage of "data" and "datum" and was intrigued by the alleged usage of it and how said usage flowed within a sentence/phrase.

It is explained here that the word data should not be used as,

...the data tells us...

but rather,

...the data tell us...

Reading the first seems natural, I've heard it quite often; however, the second seems very hard to read or say - simply unnatural in my mouth. A similar situation is found with Hoi Polloi where it is incorrect to say,

...gone to meet with the hoi polloi...

but often corrected to,

...gone to meet with hoi polloi...

due to the redundancy of the term "the" (where Hoi Polloi meaning "The Many").

While my specific usage of these two situations may not be finely crafted, I believe the concept/question to be evident: Is it better to appease my audience by using a commonly used phrasing, or is it better to adhere to the correct implementation of the words/phrases?

Another consideration is my credibility. Will the readers perhaps discredit my writings due to the unfamiliar conventions utilized?

Answer

Your best option would be to check a dictionary as it will usually have a note to clarify such ambiguities.

ODO's entry for data notes:

In Latin, data is the plural of datum and, historically and in specialized scientific fields, it is also treated as a plural in English, taking a plural verb, as in the data were collected and classified. In modern non-scientific use, however, it is generally not treated as a plural. Instead, it is treated as a mass noun, similar to a word like information, which takes a singular verb. Sentences such as data was collected over a number of years are now widely accepted in standard English.

The data/datum question has been covered on ELU before.

Similarly, the entry for hoi polloi carries the following note:

1 To those in the know, hoi is the Greek word for the definite article the (nominative masculine plural); the phrase hoi polloi thus translates as ‘the many’. This knowledge has led some traditionalists to insist that hoi polloi should not be used in English with the, since that would be to state the word the twice. Such arguments miss the point: once established in English, expressions such as hoi polloi are treated as a fixed unit and are subject to the rules and conventions of English. Evidence shows that use with the has now become an accepted part of standard English usage.