Can too much water stretch your sleeve?
Look at my response to MadlyInLoveWithCheese above and that might explain things a little better.
Greek yogurt isn't like a dense protein such as chicken and for a lot of people if it is just the yogurt without adding anything more solid food wise to it would be a "slider" food. These are foods that are either very quickly digested or somewhat liquid like and will pass through your stomach quicker than the above mentioned chicken (which is more solid).
I don't think that you would have much issue with the Greek yogurts. The string cheese maybe a little slower passing through your stomach. But really I don't think that you are causing any potential problems. It actually is pretty tough to stretch that sleeve because as I understand it the part of the stomach that they remove is actually a stretchier part than what remains.
The only problem you might experience is flushing the cheese through sooner than it would have naturally gone through or if the cheese was blocking the liquid a little bit then the liquid has only one way out and that would be to try and come back up. Although you would feel full and stop drinking before any of that would happen I would expect.
So overall I think you don't have anything to be concerned about.
http://www.obesityhelp.com/forums/VSG/4458217/The-Pyloric-Valve-How-it-works-with-the-VSG/
http://www.obesityhelp.com/forums/amos/4450030/Whats-So-Important-About-The-Pyloric-Valve-Anyway/