Ideally, you would arrive in Brazil with reais to pay for taxi and incidentals. If you know someone who would sell some reais to you, that would be a help. But, if you get an airport cab, you can use a credit card to pay the fare to town.
Even though changing money at the airport (GIG) has a higher fee than in town, the cost would not be anything approaching the cost of having a cab stop and wait for you at an ATM away from the airport. Therefore, you might want to consider getting some reais before leaving the airport.
In town, depending on which bank you use in the U.S. you will not be charged an ATM fee in Brazil. Your relationship with your bank (and, of course the particular bank) determines whether you will be charged an ATM fee.
When you use ATM machines in Brazil, inspect the slot on the ATM machine where you insert your ATM card. Look for a skimmer. A skimmer can capture your details from your ATM card. Some skimmers have a miniature camera attached which captures a photo of which numbers you use to enter your password. Skimmers are common in Brazil.
Check out this article about skimmers:
http://krebsonsecurity.com/all-about-skimmers/
With respect to paying the garotos or other services in dollars, forget about that. In my many trips to Brazil, I have never been asked for dollars by anyone at anytime.
I recently met a garoto at one of the saunas. He had a $20.00 bill an American had given to him. He was having trouble spending it or exchanging it at stores and banks. He asked me if I could change it for him into reais. No question, the dollar is not preferred in Brazil.
You also asked about Uber in Rio.
I did not use Uber in Rio on my last trip about a week ago. However, I did use Uber in Sao Paulo. I had an excellent Uber driver and the fare was about 25% less than I pay to a taxi driver from the same starting point to GRU.
Here is an article that discusses Uber in Rio. Uber was banned for a while in Rio then reinstated but I cannot find anything more current on whether you can presently use Uber in Rio:
http://www.ctvnews.ca/business/judge-issues-order-that-allows-uber-in-rio-de-janeiro-1.2604334
You might look for something more current on Uber in Rio. I could not find anything since the article that I linked above.
One more thing - - despite the warnings and cautions stated above, you will have a great time in Brazil. That is a guaranty.