"No, you are cooks."
Translation:No, voi siete cuochi.
Sei is the second person singular of the verb. There is more than one cook so the verb has to be second person plural. The plural cannot be used with the pronoun tu because it is only singular. The second person plural pronoun in Italian is voi. I know this can be an issue because we have only one second person plural in English. However, a lot of languages have different forms for number (singlular, plural) and for politeness (informal, formal). The Italian form tu is singlular and informal. It can only be used when speaking or writing to one person with whom you can be familiar.
No, it's never allowed to refer to a group of people as "tu"; you can refer to a single person as "voi", but that usage isn't uniform in Italy (and the context where it would be used in Central and Northern Italy has pretty much disappeared).
You should definitely use "tu": as you meet an Italian speaker you'll probably find both of you addressing each other as "Lei", but as you get on friendly terms with them they will most likely say "diamoci del tu" (let's address each other with "tu").
There is no codified ritual, no :) Some people are eager to shorten the distance and some to maintain an aura of respect: that's especially true if you interact with people on their job, and I remember an English lady who worked with my university told me that the staff used tu with her in person and Lei in writing, and she didn't know what to make of it. Mileage may vary. In any case, if you feel like you got close enough, you can always say "mi dia del tu", and hope that the other reciprocates to avoid creating an asymmetric relationship.
I wrote, "No, Loro sono cuochi", and do not understand why Duolingo (a) rejected it and (b) insists I write, "No, voi siete cuochi". Italian culture is very different from British or US culture. It is more conservative and language between strangers is more formal. I do not know why Duolingo forces the informal forms of 'you' on learners (which it is doing in French, German and Italian). If I ever get the opportunity to use these languages with native speakers they will be offended if I use tu, du or tu instead of vous, Sie and Lei.
Italian culture is certainly different from British or US culture, but not necessarily more formal and conservative: some native speakers will be offended if you use tu, some will if you don't, some will just laugh in your face if you pick the wrong one. This goes beyond language of course, and plenty of Italians have trouble reading the mood. As for Loro, it's an extremely rare pronoun and never actually used (only the conjugation is), so the course doesn't teach it: typically for people we address with Lei we still use voi in the plural. This post by CivisRomanus goes into more detail: https://forum.duolingo.com/comment/26798257
I believe for the most part that post says what I do. The safest thing to do if you are going to speak French, German, Italian (and probably other languages, too), is to address people, who are going to be strangers to you, in the formal second person and not the informal. Having lived in France I have first hand experience of this. A friend of mine lived in Germany and says the same about Germany. Books I have read by an American who lived in Italy for many, many years says the same about Italy. In these countries it would be more correct, and politer, to address people formally. As this is more likely to be what is required it would be more appropriate to be taught the formal forms rather than the informal ones.
Look, I'm Italian and I spent there most of my life, and I can tell you it's not so simple. As a tourist you'll find yourself using Lei often, that's true, but sometimes tu and voi as well: never Loro, unless you're rich enough to go into places most Italians wouldn't, and even then as I wrote Loro is never stated, only the conjugation is.
For some personal insight. At work I've always been in companies that used tu, and that made it pretty awkward when meeting top managers: do you address them with tu and risk seeming rude, or do you address them with Lei and contravene company policy? In doubt, I kept silent. A manager once used Lei with a barista at a local cafè: he replied in Roman "ao', anvedi questo mi sta a da' der Lei", which you could translate with "hey, look at this guy who's addressing me with Lei". Roman humour can be rude. In the end, it comes down to reading the mood; Lei for the singular and voi for the plural is a safe choice in most cases when interacting with adults, simply because it's easier for them to offer you a tu rather than request a Lei.
Then all I can do is agree to differ with you. Yours is one opinion among many. It is not part of the consensus I see. I personally would rather not cause offence.
I know Italian pronouns are often omitted because of the way verbs are conjugated. That being the case we could do with consistency on Duolingo about whether they are used or not.
I shall continue to use the polite forms of the second person in French, German and Italian and would use them if I ever get the opportunity to use these languages in a real life situation.