is not it 'tudos'?
No. tudo is neutral. todos means "all/every" for masculine words. Eles todos / Elas todas. ELES são TODOS iguais.
Os tudos deles sao mesmos - why is this wrong?
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.
"eles são todos mesmos/idênticos"?
Or you can use "Eles são todos os mesmos".
@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".
@Paulenrique, "mesmo" can be used as a noun, an adverb and an adjective: https://en.wiktionary.org/wiki/mesmo. I was using "mesmo" as an adjective as well. Why can't "mesmos" be used without "os" in here?
Thank you. 08/03/2017
The point is that I think you'll always use a definite article with the word mesmos:
Porque "são" é não "estão"
As we have no context, both "são" and "estão" should be accepted.