When you have the object/noun following the possessive, you can use "o, a, os, as", but it is opitional (sua casa = a sua casa / os seus vestidos = seus vestidos). When they are not directly related to a noun, you have to use the article: Minha casa é amarela e A sua é branca.
There is a topic about it "articles before possessives" but unfortunately i'm not being able to copy and paste. =( try to find it if podssible. But, as you said, when you have mine, yours, theirs etc you need to use the definite article, but opitional for my, your, her, etc.
According to grammar rules, "seu" is more formal and agrees with the pronoun "você"; "teu" is used with close people or informal situations and agrees with "tu". But in Brazilian Portuguese is common to always use "você" and "seu/sua/seus/suas" in any type of situation. "Seu/sua" can mean "your" or "his/her"; "teu/tua" is just "your"