In the flashback at the Plumbers Academy a pair of plumbers somewhat resembling the Mario Bros rivals Wario and Waluigi are seen. One, looking exactly like Wario is wearing Wario's yellow/purple outfit and has a mustache, the other next to him has no mustache but is taller and has a long nose, a green/light green outfit.
Though these cartoons were made 5 years or so before Wario's debut in Super Mario Land 2: 6 Golden Coins, and years before Waluigi appeared in Mario Tennis on N64(not to mention the Mario cartoons have never been made officially "Canon" in the Mario franchise/history), it could very well be Wario and Waluigi.
During the show's original run, this segment was considered infamous due to the song cover "Workin' For A Livin'" had the profane word "damn" used three times. Due to this and copyright restrictions, the song is removed for syndication.