It's a given in the Toobworld concept that even though a show may be cancelled, its characters continue leading their lives in the TV Universe. And it doesn't matter how many years pass since they were last seen - take the cast of 'The Dick Van Dyke Show Reunion', for example. With the exception of Alan Brady, who appeared in an episode of 'Mad About You' back in the 1990s, most of them had not been seen since 'The Dick Van Dyke Show' left the air.
(Rob and Laura Petrie, shown here from the reunion special, appeared together in a sketch for a 1969 TV special that reunited Van Dyke with Mary Tyler Moore. But those Petries would have been doppelgangers from Skitlandia.)
When 'Stargate SG-1' finished its run, Samantha Carter moved over to the sequel, 'Stargate: Atlantis'. And Teal'c has now made two appearances on the series - the latest being in the recent episode "Midway".
Because of their different line of succession for the Presidency of the United States, the 'Stargate' series on the whole have to be relegated to a different dimension. However, It could be that the series started out on Earth Prime-Time and they "jump-gated" back to a new dimension with nobody the wiser. And then the national elections caused the divergence.
At any rate, I think Teal'c does exist in Earth Prime-Time, and it would be great if we got the chance to see him show up on another sci-fi series someday......