Is "nos" necessary?
No, it isn't. Sumus contains the full idea "we are." The pronoun nos could add emphasis but often would be emitted.
Now the word forum is translated into english as forum as well whereas before was used in the examples as Market.
Both is be accepted normally (when it's not: flag!), as the forum is also a place, everything that can be bought in a market, can be in a forum.
Both are accepted normally... ,:)
Is it correct to translate "we are in the square"?