Learn English – People who tell me vs. people telling me

It sounds to me like people are just leaving out words, as opposed to completely violating the grammar. This often happens in conversation.

• "You fighting?" -> "Are you fighting?" (as in a continuous disagreement instead of a momentary battle)
• "You tell her?" -> "Did you tell her?"
• "You be there?" -> "Will you be there?"

It seems like people are just leaving out the helping verbs. It's certainly not proper but it's acceptable in casual conversation.

As for whether or not Miamians speak this way, I can't say, but I'd guess lots of native English speakers will speak this way from time to time. I'm in Canada and I would not find any of this speech surprising or difficult.

Your question is somewhat misleading, and may contain a false assumption. A statement like the following:

Customers who have never paid a dime

is not a sentence. It's a noun phrase. Let's break that down:

Customers is a noun. Nouns are not inherently subjects or objects, so it's not correct to refer to this as either the subject or the object.

The remaining part who have never paid a dime is a relative clause, which is an embedded sentence that is used to modify something in the larger context. The subject of the embedded clause is the relative pronoun who, and the remainder have never paid a dime is the predicate of the relative clause.

However, using terms such as "predicate" and "relative clause" may be intimidating to people using your software. My best offering for newbie-friendly terminology would be:

• Call Customers a noun (most people at least know what that means)
• Call who a "question word", and make it clear by example what you're referring to
• Call have never paid a dime a "description"