Before possessive adjetives (with a following noun), using the article it's optional in Brazil (but preferred in Portugual)
Notice that when there is no following noun, you can change the meaning:
- São meus = They belong to me = They are mine
- São os meus = (hard to translate exactly) they are the ones that belong to me
Although "tu" might be regionalized, its related pronouns (teu, te, ti, etc.) are omnipresent.
In Brazil, there isn't really a distinction between formal and informal.
Of course there is a trend of using "tu + conjugation for você" - now this is informal of course. It's wrong but very common.