In feudal Japan, rats (nezumi) were the symbol for wealth. Because feudal Japan was a rice based economy with actual taxes paid in rice, when the grain bins are full (bountiful harvest), rats are found in greater numbers around the bins.
I don't think there's anything wrong with using ratfolk as an unnecessarily ostracized culture that may not conform to human opinion of what rat tropes exist in western society. In fact it may be a positive way that young players can play as underdogs in society, and a way to prove themselves as a positive member of urban society - a good lesson on appreciating those who receive societal prejudice. Ratfolk would be extremely family oriented with strong bonds between multiple generations. They see the value in what others dispose of before their effective usefulness is over. They might be matriarchal, thus more concerned with the betterment of the clan/colony, than being intrusive.
Perhaps they can help runaway children who've sought refuge in the sewers and now face more dangerous threats that the ratfolk intend to rescue and encourage them to return home - at least for the sake of their family. Have a party of PC ratfolk leading the children throughout a more extensive sewer system to return them closest to their homes, with fights, hit and runs, sneaking to avoid homeless muggers, sewer crocodiles, green slime, and other denizens of the sewers.