Shrimp do not eat feces, but will help clean up plant debris, small growths of algae, and uneaten fish food.
Species of Palaemonetes (ghost shrimp), Atya (fan shrimp), and Macrobrachium (whisker or longarm shrimp) are native to South America. Angels would probably pick off ghost shrimp pretty quickly, and the larger Macrobrachium species would be a threat to the fish. Atya gabonensis is a filter feeder and should not be kept for tank maintenance, but is a cool species in its own right. I suggest smaller Macrobrachium species as a 'clean-up' crew.
One thing to be aware of is that crustaceans generally don't do well in acidic conditions. If you set this up as a blackwater tank you should probably forego shrimp altogether.