Not to sound patronizing but I would assume going to any "poor" neighborhood in a place like Rio is a lousy idea for a bunch of green foreigners. Poor people are not necessarily bad. I've traveled a bit in third-world countries and it is not a "bad" or dangerous experience if you understand what you are doing. But one of the things I've learned is that urban slums should always be avoided if possible.

Traveling in the countryside is often different because the folks that live there, as poor as they might be, tend to bo friendlier, sometimes surprisingly hospitable and in many parts of the world they will actively protect any foreigners as long as they're not "dumb Americans" flashing their money and driving huge SUVs. There is always a chance of running into bad guys but as long as you're not ignorantly passing through a war zone or getting mixed up in a guerilla uprising you should be reasonably safe.

In big cities and slums though moral principles are often nonexistent, pretty much everything goes. You could literally get killed on the spot for a few dollars or just a pair of sneakers. But any reasonably experienced traveller should be able to identify such danger zones easily enough if only you pay attention to what is going on around you. One fairly reliable sign is the sudden lack of police presence. In many places bordering on dangerous slums you will see lots of heavily armed police protecting the safe areas, usually where decent (or at least rich) people live. A few blocks further down the road and you might end up in a hellhole where not even a police patrol would dare to enter.

Of course, if you happen to be too busy to maintain some basic situational awareness and prefer to take directions from the GPS you might as well find yourself in a really bad spot.