No, Rio Negro plec's won't help and many plec's in general are not great about eating algae. As already stated if you correct the problem causing the algae you won't need to worry about something that may eat it. Algae eaters tend to eat only certain types of algae and there are some algae that none will eat.
If your having green dust algae and diatoms your tank is obviously newly set up and going through that phase. Only run lighting 6 hours daily which will help keep algae growth down. Diatoms are self limiting and will go away once excess silicates in the water are exhausted. Green dust algae will also run its course and go away on its own in time. Nerite snails, which can't reproduce in freshwater are the best for keeping excess of both types cleaned up. Then use an algae magnet to keep the front and side glass cleaned up.
Also if this is a fish only tank you can use phosphate remover for 2-3 weeks which will remove silicates as well which will kill off diatoms.