Learn Portuguese (Portugal): Moramos juntos desde março.

In Portuguese, when an action started in the past and is still happening right now, you usually use the present tense with desde (since). You literally say "We live together since March" instead of using a complex past tense like we do in English.

Would juntos ever change to juntas?

Yes! The word juntos (together) has to match the gender of the people it describes. If all the people in the "we" group are female, you must use the feminine plural juntas. Since it is juntos here, the group is either all male or a mix of men and women.

Is it okay to add Nós (we) at the beginning of the sentence?

Yes, you can say Nós moramos juntos desde março, but it is completely optional. The verb ending in -amos already makes it perfectly clear that the subject is "we", so native speakers usually just leave nós out.

Why isn't the month março capitalized?

In Portuguese, the names of the months and days of the week are always written with lowercase letters, unless they happen to be the very first word in a sentence.