Casey Kasem was the voice of Alexander. Since Casey Kasem also voiced Shaggy, the crossover to The New Scooby-Doo Movies resulted in a lot of Talking to Himself. That was particularly weird since, although Kasem is perfectly capable of doing a wide range of voices, he had voiced Shaggy and Alexander pretty much the same way.
Melody's singing voice was by Cheryl Ladd (her speaking voice was Jackie Joseph, then of The Doris Day Show).
The Pussycats were originally meant to appear as teammates of the Scooby Doobies on Laff-A-Lympics, but clearance issues with Archie Comics' imprint Radio Comics prevented it. They were replaced with Captain Caveman and the Teen Angels.