Don't think too much about the "why" of gender in Portuguese. The fact is that gender assignment to terms is NOT systematic. You will waste your time if you try too much to puzzle this one out. Don't worry, just be happy!
Can´t you say ¨meu vestidos¨
No. "Meu" is used for singular words and "meus" for plural words. (Meus vestidoss)
Can anyone help with when I use "Meu/Meus" and when I would use "Minha/Minhas" ? Can they ALL me used to express Mine, My (sing) ? Thanks