I used to live on the South Shore (but now live in CO). I think you'd enjoy the Chili Bar section of the S. Fork of the American. There are usually a ton of boaters at the put-in and you should have no problem finding people to boat with and making paddling friends from there.
Also pretty fun (but definitely in the Class IV category) is the Kyburz section of the S. Fork of the American. It's in the town of Kyburz on hwy 50 between SLT and Placerville. That was my go to after work run back in the day.
You should also buy the Hobeck/Stanley CA guidebook. Any paddling shop should have it or you could buy it online. Here is the link: Amazon.com: Best Whitewater in California: The Guide to 180 Runs (9780966528909): Lars Holbeck: Books
Good luck and have fun out there.