Positions of sim:
"Eu sim falo português" - Sim is related to eu. It suggests someone else doesn't speak, but in contrast, I do.
"Eu falo português sim" - Sim is related to falo. It means: I DO speak Portuguese
Would it be correct if I said: "falo portugués" ; without having to use "Eu".
In Spanish, for example, you can just say "hablo español". You don't need to use "Yo"
Yes, it would :)
But for "ele/ela/você", it could get a little confusing if you omitted the pronoun, because the conjugations are the same. In more complete contexts, it's ok too.
I'm assuming that you mean something along the lines of: "Eu sim falo portugues">> I DO speak Portuguese. "Eu falo portugues sim">> I speak Portuguese, yes. Is that a reasonable breakdown?
Fala português? Falo. (Yes, I do)
Fala português? Falo, sim. (Yes, I do speak Portuguese) emphatic
The Grammar of Spoken Brazilian Portuguese - Earl Thomas - Vanderbilt U Press
In certain circumstances, I have seen portugues with an indefinite article before it. Is this correct, and if so, when should I use it?
It's how the verb is conjugated. Think about English - you say "I speak" but "He speaks" - the verb changes to agree with the subject. Although in English there are very few conjugations ("speak" is the same form for "I", "you", "we", and "they"), in Portuguese, the verb changes for just about every person.
Conjugation of falar in the present tense:
Eu falo (I speak)
Tu falas (you speak)
Você fala (you speak)
Ele/Ela fala (He/she speaks)
Nós falamos (We speak)
Eles/Elas falam (They speak)
Vocês falam (you-plural speak)
It's outdated. Nowadays people use "vocês" or "Os senhores/As senhoras" (formal).
Is this just in Brazil or is it also in Portugal and other areas that speak Portuguese? I heard that it was still in use over there.
"Vós falais" (you all speak)
Usually used in written texts, special in very formal ones (like in the Holy Bible)...