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человечек is a word of masculine gender and it requires masculine form of its attributives even if it refer to the female person.



I'd say that it is not a common way. But among close friends...

Yes, [Ты] моя хорошая. = You are my good (literally). "моя хорошая" describes "Ты" and requires corresponding gender. Grammatical gender depends on the grammatical relations in a sentence.

Nope. She should say "моя хорошая" instead. It is the same but with feminine grammatical gender.