Zorak's race is enslaved after his capture/death.
In this series we see Zorak-like aliens that work for a much smaller alien invader. Clearly, after Space Ghost ended Zorak's reign of terror on the galaxy, Zorak's race is easy picking for a much smarter conqueror. That conqueror then moved his sights to Earth and had his run-in with the Neptunes later in history. Jabberjaw is set in the future of SeaQuest DSV.
It makes sense that the only two TV series to deal extensively with colonization of the ocean floor would be set in the same verse, though Jabberjaw is clearly set later, because the tech is more advanced (time machines and such). Jabber is a genetically engineered Uplifted Animal, grown with biotech well beyond what was used to create the Daggers.- Either Sea Lab 2020 or Sea Lab 2021 may fit early in the timeline as well.
Clamhead is a descendant of Shaggy
Given their looks and similar roles it isn't impossible Clamhead is the latest in the Rogers line by the time of Jabberjaw's future. Volton is a descendant of Mr. Voltner from The Scooby-Doo Show
Volton's electric form greatly resembles Voltner's 10,000 Volt Ghost costume. After his defeat and arrest Voltner and his family used their skills to advance the technology to the point his descendant has gone from "Scooby-Doo" Hoax criminal to legitimate super villain. Jabberjaw stole the time machine
After the events of the series ended, Jabberjaw having gotten no respect went off solo into the "past" where he would have all this Crossovers without his friends.