Yes, they could! That's the fun part! Including the one where Thor becomes a member of SG-1 while Carter goes off to study on the Asgard homeworld. Don't worry, it only lasts a season.
Why do you think the show got canceled? Thor was a horrible replacement, the audience just didn't connect. Then Sam gets shipped off to Atlantis to have Lemonhead's baby.
And Teal'c was already replaced with evil Teal'c. Where do you think the hair came from...
I thought they left it uncertain which Teal'c we wound up with, like Sliders did with Professor Arturo. So the real Teal'c could still be out there.
What did you think about Emmett Bregman becoming the leader of SG-17? I think it was a ballsy move. A filmmaking SG team has a lot of story potential.