All the Hanna-Barbera shows take place in one universe.On a trip to Europe, Dr. Benton Quest discovered medieval texts telling of small blue creatures of human-like intellect. He eventually runs into a live specimen as well as a similar race of aquatic beings described in 17th-century naval reports. Dr. Quest creates intelligent talking animals by mixing Smurf and Snork DNA with various species. But many of the test subjects, such as Yogi Bear, Huckleberry Hound, Magilla Gorilla, Scooby-Doo and Top Cat's gang escaped into the wilds or into the cities. As these animals grew more intelligent, some like Hong Kong Phooey, Quick-Draw McGraw, Atom Ant and Secret Squirrel would help humans in keeping the peace as superheroes or secret-agents. Government genetic engineering efforts to create their own heroic talking animals, robots and superheroes ranging from Dynomutt to The Impossibles and Frankenstein Jr. met with mediocre success or just making things worse. When The Valley Of The Dinosaurs was discovered in South America, the U.S. government rushed to create more intelligent creatures, though only resulting in rather docile prehistoric experiments. As things on the surface continued to get worse, humanity's elite bailed from the surface, ranging from going into towering ivory "magic sky castles", under the sea or into space. The people and intelligent animals, prehistoric and otherwise, that were left behind reverted to a more prehistoric culture that still resembled the lost ways—a modern Stone Age, if you will—followed by a slightly less ancient culture. As humanity declined, the more human-like talking animals would create their own civilizations resembling those of humanity before the fall including Megakat City and an undersea city with the law being kept by Fish Police. Laff-A-Lympics, Yogi's Gang and Wacky Races confirm this for early Hanna-Barbera.
All the Hanna-Barbera shows take place in one universe w/ better research and logicHanna Barbera shows did not have a set rule, but has the standard logic that we can see stand true back at MGM, through their own and into some modern attempts at them. All HB shows exist in one universe, but it is a faulty timestream. The amount of characters who time travel or warp reality are numerous (Great Gazoo, Shazzan, Jeannie and Babu, the Pastmaster, the smurfs in the last season, Cupcake, plus one time baddies like Dr Millenium, Time Slime, Timeatron, etc) that if one takes the assumption they all co-exist in order to crossover, then all that damage to the timeline has created many cracks and issues that resulted in times where canon meant something and times where it was irrelevant because the timeline was changed without anyone realizing. This at it's bare minimum creates reasonable excuses for every continuity problem there ever was or ever will be. This creates the plausible world and why these cartoons sometimes have zero regard for continuity but sometimes pull it out for pure brilliance (the Herculoids using a ship they captured earlier in the series, the Smurfs being one short of 100 which later revealed because they lost wild smurf, a pup named scooby doo bringing back Shaggy and scooby's sisters, snagglepuss's earlier antics being blamed on his brother snaggletooth that shows up in a snagglepuss short, etc)
Cartoon Network bubble?In addition to the above, perhaps the real life Turner buyout was a crisis crossover to the Hanna Barbera world? They exiled cartoon network's characters out of their world but the result ripped shells of their former selves into the cartoon network and adult swim shows. Also explaining why those shows often feature cameos of older cartoons yet the modern continuations of the Hanna Barbera shows do not return the favor with cartoon network references.