"Are they all the same?"
Translation:Eles são todos iguais?
No. tudo is neutral. todos means "all/every" for masculine words. Eles todos / Elas todas. ELES são TODOS iguais.
that just does not make sense in Portuguese. Maybe "eles são todos os mesmos?"
Is "eles" only used in this sentence when talking about people or would it be the same for instance for chairs?
You use eles/elas for objects in plural too. As cadeiras are feminine, use elas.
@Paulenrique, why you add "os" before "mesmos"? Why don't "idêntico" and "igual" need "os" before them? You are considering all the three words as adjectives or nouns?
Because "iguais" and "idênticos" work as adjectives here, but it is not the case for the word "mesmos", so you need to add "os".
The point is that I think you'll always use a definite article with the word mesmos:
- Eles são os mesmos rapazes.
- Eles são os mesmos.