Seven action-packed days is a good start!
Rio offers many public transit options, including bus and metro. Note that air-conditioned buses cost a little more than non-air-conditioned buses, so if you are traveling a long distance around the city and you're not used to hot weather, it may be worth considering shelling out a bit more. It's quite a hilly city, so walking can sometimes be akin to hiking - make sure you are wearing good shoes.
All year round - the temperature does not vary too much.
Rio's beach etiquette may be intimidating at first, but you're meant to enjoy yourself! Note that it's forbidden to bring your own food, and there may be separate sections on the beach for nude bathing.