All yall is not usually correct, even in southern english. Yall is already plural, there is no need to use all in front of it except in very specific cases where you are addressing multiple groups of people (yall 2 go one way, yall 3 go another, yall 5 go a third way, all yall meet back up in 10 minutes).
As I understand it, you should address family, friends, and closer aquaintainces with "tu", while strangers and professionals should be addressed with "voi", which also gets used for a group of people.
It seems to me that you probably say "voi" more than you say "tu" during a day, especially if you are a tourist in Italy. I think that Duolingo should put more phrases using the "voi" conjugations (such as this example- "siete") into these exercises. I think that they lean on "tu" a little too much, since I won't be speaking Italian to my own family.
Just a thought; not really sure where I should put it otherwise!
"Voi" is also informal. It is the plural of "tu." There's a brief explanation here:
(If you know any Spanish, it is the equivalent of "vosotros" in that language.)
The formal second person [you] pronouns, Lei (singular) and Loro (plural) look just like the third person [he, she, it, they] pronouns, lei (singular) and loro (plural) except that the "you" forms are always capitalized.
Native speakers out there -- HELP! Is it as impolite to address a stranger as "tu" in Italian as it is in French or Spanish (unless you are both very young)?