If you want to, I guess. A lot of Spanish learning aides, especially aimed at beginners, give ustedes as "you-all" or something of the like; the fact is, however, that "you" is both singular and plural in English and "you all" sounds a bit silly and superfluous. If you need to disambiguate, saying something like "you guys," "you people" or in this case "you chefs" or something is better.
Yes, that is what "ustedes" means. You all. Its referring to plural "you". But, you don't NEED to say "you all"
:D however "tú" does not have a plural form per se. In Spain it's common to say vosotros which is plural "you" in the informal register.In 99% of Latin America, vosotros is said by no one and "ustedes" is used, along with its matching conjugations, for both formal and informal situations.