I don't think you're being stupid, no. I agree that it's an informal term... I don't know of anyone who would disagree, but maybe that has something to do with where I live. I use y'all and "you guys" (or yinz when I want to be obnoxious) when I want to pluralize "you," but I never use it in a formal context, as a rule.
But I do kind of wish that English had some form of plural "you," like other languages. I feel like we're missing out.