Can plecos + corydoras actually swim?

They can swim as any other fish can, from point A-B-C, but Plecos and catfish aren't going to swim as "graceful" as some other fish because that isn't their purpose. Their bodies are shaped just so, so they can adhere to the tank wall and clean it..or any other surface. The catfish's purpose is the scour the tank floor, scavenging for leftovers. That's why they are also "flat" on the underside. Now, I have Upside-down Catfish and I have no idea why they swim like they do! *laughs* but it's fun to watch.